<PAGE>
 
                                                                  EXHIBIT 10.51


                         FACILITIES LEASE AGREEMENT

                               BY AND BETWEEN

                    LCOR CLARKSBURG L.L.C., AS LANDLORD

                                    AND

                       COMSAT CORPORATION, AS TENANT





                  CLARKSBURG, MONTGOMERY COUNTY, MARYLAND
<PAGE>
 
                             TABLE OF CONTENTS




         INTRODUCTION
         Section 1.      DEFINITIONS........................................1
         Section 2.      LEASE OF INSTALLATIONS PREMISES....................2
         Section 3.      TERM...............................................2
         Section 4.      RENT...............................................3
         Section 5.      USE OF THE INSTALLATIONS PREMISES..................5
         Section 6.      DEVELOPMENT; DESIGNATION OF THE EXCLUDED
                              AREAS.........................................7
         Section 7.      TAXES AND IMPOSITIONS..............................9
         Section 8.      UTILITIES.........................................12
         Section 9.      SIGNS.............................................13
         Section 10.     AS-IS CONDITION OF INSTALLATIONS
                              PREMISES.....................................13
         Section 11.     REPAIRS, MAINTENANCE AND MANAGEMENT...............14
         Section 12.     ACCESS TO INSTALLATIONS PREMISES..................15
         Section 13.     ALTERATIONS AND PERSONAL PROPERTY.................16
         Section 14.     INSURANCE.........................................17
         Section 15.     DAMAGE OR DESTRUCTION.............................20
         Section 16.     INDEMNIFICATION...................................21
         Section 17.     CONDEMNATION......................................25
         Section 18.     LIENS.............................................26
         Section 19.     EXISTING SPACE LEASES; ASSIGNMENT AND
                              SUBLETTING...................................27
         Section 20.     SUBORDINATION OR SUPERIORITY OF LEASE.............28
         Section 21.     DEFAULTS AND REMEDIES.............................29
         Section 22.     BANKRUPTCY OR INSOLVENCY..........................34
         Section 23.     SURRENDER OF INSTALLATIONS PREMISES...............36
         Section 24.     NON-CONSENSUAL HOLDING OVER.......................36
         Section 25.     QUIET ENJOYMENT...................................37
         Section 26.     NOTICES...........................................37
         Section 27.     HAZARDOUS MATERIALS...............................38
         Section 28.     RIGHT TO RENEW TERM...............................40
         Section 29.     SECURITY DEPOSIT..................................41
         Section 30.     MISCELLANEOUS GENERAL PROVISIONS..................41
         SCHEDULE I
         SCHEDULE OF EXHIBITS
<PAGE>
 
                         FACILITIES LEASE AGREEMENT


                  THIS FACILITIES LEASE AGREEMENT (this "Lease") is entered
into  and  made  effective  as of the  12th  day of  September,  1997  (the
"Effective  Date"),  by and  between  LCOR  CLARKSBURG  L.L.C.,  a Delaware
limited liability company  ("Landlord") and COMSAT Corporation,  a District
of Columbia corporation ("Tenant").

                                WITNESSETH:

                  (a) Prior to but contemporaneously with the execution and
delivery  of this  Lease,  Tenant sold and  transferred  the  Installations
Premises to Landlord  pursuant to that certain  Agreement of Sale dated the
Effective  Date,  by and  between  Tenant,  as  seller,  and  Landlord,  as
purchaser (the "Purchase and Sale Agreement").

                  (b)  Landlord  desires  to lease to  Tenant,  and  Tenant
desires to lease back from Landlord, the Installations Premises, all as set
forth and on the terms and conditions contained in this Lease.

                  NOW THEREFORE,  in  consideration  of the mutual promises
set forth in this  Lease and other  good and  valuable  consideration,  the
receipt and sufficiency of which are  acknowledged by each party,  Landlord
and Tenant agree as follows:

                  SECTION 1.        DEFINITIONS

                  (a) The following terms shall have the meanings  ascribed
thereto below:

                  Agents:  The agents, employees, contractors,
subcontractors, affiliates, licensees and invitees of each party
(and in the case of Tenant, subtenants).

                  Auxiliary  Buildings:  All  buildings  now  or  hereafter
located on the  Installations  Premises and  currently  consisting of three
buildings which, for the purposes of this Lease,  Landlord and Tenant agree
contain, in the aggregate, 36,000 gross square feet of space.

                  Communications Facilities:  The satellite dishes,
antennae, or similar structures or equipment now or hereafter
located on the Installation Premises.

                  Expiration Date:  The ending date of the Term.

                  Installations  Premises:  Certain portions of the Land as
described  and shown on Exhibit C attached  hereto and made a part  hereof,
and containing the Auxiliary Buildings and the Communications Facilities.


<PAGE>
 
                  Land:   Approximately   227  acres  of  land  located  in
Clarksburg, Montgomery County, Maryland, as described or shown on Exhibit A
attached hereto and made a part hereof.

                  Master Lease Property:  The Land, together with all
improvements located thereon, less the Installations Premises and
the Excluded Areas.

                  Main  Building  Area:  The Main  Building and the Parking
Areas,  access drives, and other appurtenant areas serving the Buildings on
such area,  as described  or shown on Exhibit B attached  hereto and made a
part hereof.

                  Parking Areas:  Those certain areas located on the
Installations Premises and designated for use for parking of
motor vehicles as of the Effective Date.

                  Unimproved Area:  The Master Lease Property, less the
Main Building Area.

                  (b) Other  terms  shall have  meanings  ascribed  to such
terms in this Lease and as shown on  Schedule I attached  hereto and made a
part hereof.

                  SECTION 2.        LEASE OF INSTALLATIONS PREMISES

                  Landlord,   for  and  in   consideration  of  the  rents,
covenants and agreements  hereinafter reserved,  mentioned and contained on
the part of Tenant and its  successors  and assigns,  to be paid,  kept and
performed,  has leased, rented, let and demised, and by these presents does
lease,  rent,  let and demise unto Tenant,  and Tenant does hereby take and
hire,   upon  and  subject  to  the  covenants,   agreements,   provisions,
limitations and conditions herein expressed, the Installations Premises.

                  SECTION 3.        TERM

                  (a) The initial term of this Lease (the  "Initial  Term")
shall be for a period of ten (10) years  commencing on the  Effective  Date
and  ending  at  11:59  p.m.  local  time on the day  preceding  the  tenth
anniversary  of the  Effective  Date,  unless  this  Lease  shall be sooner
terminated as hereinafter  provided or as provided by law.  Notwithstanding
the  foregoing,  if the Term ends on any day other than the last day of any
calendar month, the Term shall be extended so that the last day of the Term
is the  last  day of such  calendar  month.  The  Initial  Term,  as may be
extended by exercise of Section 28, may sometimes be collectively  referred
to in this Lease as the "Term".

                  (b) As used in this Lease,  the term  "Lease  Year" means
each  consecutive  period of twelve (12)  calendar  months during the Term,
commencing on the Effective Date. If, however,  the Effective Date is other
than the first day of a calendar month, the first Lease Year shall begin on
the Effective Date and end on


                                                         
<PAGE>
 
the first  anniversary  of the last day of the calendar  month in which the
Effective  Date  falls,  and  each  succeeding  Lease  Year  shall  be each
succeeding  consecutive  period of twelve (12) calendar  months  thereafter
during the Term.

                  SECTION 4.        RENT

                  (a) Tenant covenants and agrees to pay Landlord in lawful
money of the United States, annual base rent for the Installations Premises
("Base Rent") in twelve (12) equal monthly installments,  in advance, on or
before  the first  day of each and  every  month  throughout  the Term,  as
follows:

                  (i) FOUR HUNDRED TWENTY-FIVE  THOUSAND AND NO/100 DOLLARS
($425,000.00) during the first Lease Year.

                  (ii)  FOUR  HUNDRED   THIRTY-SIX   THOUSAND  SIX  HUNDRED
EIGHTY-SEVEN  AND 50/100  DOLLARS  ($436,687.50),  during the second  Lease
Year.

                  (iii)  FOUR  HUNDRED  FORTY-EIGHT  THOUSAND  SIX  HUNDRED
NINETY-SIX AND 41/100 DOLLARS ($448,696.41), during the third Lease Year.

                  (iv) FOUR  HUNDRED  SIXTY-ONE  THOUSAND  THIRTY-FIVE  AND
56/100 DOLLARS ($461,035.56), during the fourth Lease Year.

                  (v) FOUR HUNDRED  SEVENTY-THREE  THOUSAND  SEVEN  HUNDRED
FOURTEEN AND 04/100 DOLLARS ($473,714.04), during the fifth Lease Year.

                  (vi)  FOUR  HUNDRED  EIGHTY-SIX  THOUSAND  SEVEN  HUNDRED
FORTY-ONE AND 18/100 DOLLARS ($486,741.18), during the sixth Lease Year.

                  (vii) FIVE HUNDRED  THOUSAND  ONE-HUNDRED  TWENTY-SIX AND
56/100 DOLLARS ($500,126.56), during the seventh Lease Year.

                  (viii)  FIVE  HUNDRED  THIRTEEN  THOUSAND  EIGHT  HUNDRED
EIGHTY AND 04/00 DOLLARS ($513,880.04), during the eighth Lease Year.

                  (ix) FIVE HUNDRED TWENTY-EIGHT THOUSAND TWELVE AND 10/100
DOLLARS ($528,012.10), during the ninth Lease Year.

                  (x)  FIVE  HUNDRED   FORTY-TWO   THOUSAND   FIVE  HUNDRED
THIRTY-TWO AND 43/100 DOLLARS ($542,532.43), during the tenth Lease Year.

Notwithstanding  the foregoing,  if the Effective Date shall be a day other
than the first day of a calendar  month,  there shall be due and payable on
the Effective  Date, as the  installment  of Base Rent for such  fractional
month,  an amount  determined by dividing the Base Rent for the first Lease
Year by 365 and multiplying the
<PAGE>
 
result by the number of days from the Effective Date through the
end of such month.

                  (b)  Tenant  also   covenants   and  agrees  to  pay,  as
additional rent (the  "Additional  Rent"),  all sums,  Impositions,  costs,
expenses and other  payments  which Tenant in any of the provisions of this
Lease  assumes,  agrees  or is  obligated  to pay,  or which  shall  become
otherwise  due and payable from Tenant to Landlord  under this Lease (other
than  Base  Rent).   Base  Rent  and  Additional   Rent  may  sometimes  be
collectively referred to herein as "Rent".

                  (c) It is the purpose  and intent of Landlord  and Tenant
that,  except as  explicitly  set  forth  herein,  the Base  Rent  shall be
absolutely  net to  Landlord,  so  that  this  Lease  shall  yield,  net to
Landlord, the net annual rent specified in Subsection (a) of this Section 4
in each  Lease  Year  during  the Term and that  all  costs,  expenses  and
obligations  of every kind and nature  whatsoever,  in  connection  with or
relating to the  Installations  Premises  shall be the obligation of Tenant
and shall be paid by Tenant.

                  (d) The Base Rent shall be paid to Landlord promptly when
due without notice or demand therefor, and without any abatement (except as
explicitly  stated to the contrary in Section 8),  deduction or set-off for
any reason whatsoever.

                  (e) No  payment by Tenant or  receipt  or  acceptance  by
Landlord of a lesser  amount  than the  correct  Rent shall be deemed to be
other than a payment on account,  nor shall any endorsement or statement on
any check or any  letter  accompanying  any check or  payment  be deemed an
accord and  satisfaction,  and  Landlord  may accept  such check or payment
without  prejudice to Landlord's right to recover the balance or pursue any
other remedy in this Lease or as provided by law or in equity.

                  (f) If any  of the  Rent  payable  under  the  terms  and
provisions  of this  Lease  shall be or become  uncollectible,  reduced  or
required to be refunded  because of any rent  control or similar act or law
enacted by a valid  governmental  authority,  Tenant  shall enter into such
agreements  and take such other steps that  Landlord may request and as may
be legally  permissible  to permit  Landlord to collect  the maximum  rents
which,  from  time to time  during  the  continuance  of  such  legal  rent
restriction,  may be legally  permissible (and not in excess of the amounts
reserved  therefor  under this Lease).  Upon the  termination of such legal
rent  restriction,  (i) the Rent shall become and  thereafter be payable in
accordance with the amounts reserved herein for the periods  following such
termination, and (ii) Tenant shall promptly pay to Landlord, to the maximum
extent  legally  permissible,  an amount  equal to (a) the Rent which would
have been paid pursuant to this Lease but for such legal rent  restriction,
less (b) the Rent paid by Tenant  during the period  such rent  restriction
was in effect.
<PAGE>
 
                  (g) All Rent and other  payments  required  to be made by
Tenant to Landlord shall be delivered to Landlord by wire transfer pursuant
to the wire transfer instructions as shown on Exhibit E attached hereto and
made a part hereof or to any other single  party that  Landlord may specify
from time to time by written notice given to Tenant.

                  (h)  In  recognition  of  the  extra  costs  to  Landlord
resulting from Tenant's  failure to make timely payment of any  installment
of Base Rent,  if any such  installment  is not paid  within  nine (9) days
after its due date,  the  delinquent  amount  shall be subject to a service
charge of five  percent  (5%) of such  delinquent  amount,  or such  lesser
charge as may be the maximum charge  permitted by law. In addition,  if any
installment  of Base Rent or any other sum due  Landlord  under  this Lease
remains unpaid  seventy-five  (75) days after its due date, the outstanding
amount  shall bear  interest at an annual rate of two percent (2%) over the
"Prime Rate" then prevailing or such lesser rate as may be the maximum rate
permitted by law (the "Stipulated  Rate"), and calculated from the due date
of such sum and  continuing  through the date such sum is paid in full.  As
used in this Lease,  the term "Prime Rate" means the prime rate of interest
for large money center banks as published in the Money Rates section of the
Wall Street  Journal or if the Wall Street  Journal  ceases to publish such
rate, as established  by reference to such other  authority as is generally
accepted in the business  community as a source for  determining the "Prime
Rate".

                  SECTION 5.        USE OF THE INSTALLATIONS PREMISES

                  (a) The  Installations  Premises may be occupied and used
for any  lawful  purpose  and shall not be used for any other  purpose.  In
addition, Tenant shall have easements on, over, across and through the Main
Building Area and the Unimproved  Area for (i) access to the  Installations
Premises via roads  existing in the  Unimproved  Area and the Main Building
Area as of the Effective Date, or as may be relocated as expressly provided
for in  this  Lease,  (ii)  purposes  which  are  incidental  to use of the
Installation Premises, such as use, maintenance, repair, and replacement of
utilities,  storm  drainage  facilities,  and  signage  serving  or used in
connection with the  Installation  Premises,  (iii)  reasonable  recreation
uses,  such  as use of  the  existing  recreational  facilities,  and  (iv)
temporary  activities  and uses that  Tenant  reasonably  deems  desirable.
Notwithstanding  anything to the  contrary  contained  in this Lease,  with
respect to the uses  described  in clauses  (iii) and (iv)  above,  (y) the
Tenant shall ensure that such uses do not interfere  with the  Development,
and (z) no action or omission of Landlord,  or its Agents,  successors  and
assigns that interferes with such uses shall be a default of Landlord under
this Lease.
<PAGE>
 
                  (b)      Tenant further agrees as follows:

                           (i)      Tenant and its Agents shall use the
Installations  Premises  and  conduct  their  business  thereon  in a safe,
careful, reputable and lawful manner.

                           (ii) Tenant shall obtain, or cause to be obtained,
all certificates, licenses and permits necessary for its, or its subtenants',
occupancy, use, operation and maintenance of the Installations Premises. Upon
reasonable advance request by Tenant, Landlord shall reasonably cooperate with
Tenant in Tenant's obtaining such necessary certificates, licenses or permits,
including, without limitation, signing applications for the same within ten (10)
business days after Tenant's request if Landlord's signing is required by
Applicable Law or requested by a governmental authority; provided, that Landlord
shall not be required to incur any out-of-pocket costs to third parties in
connection therewith. Tenant shall promptly reimburse Landlord for any of its
reasonable out-of-pocket costs to third parties for review or advice about such
certificates, licenses or permits.

                           (iii) Tenant shall not commit, nor allow to be
committed,  in, on or about the Installations  Premises,  any act of waste,
including  any act which might  deface,  damage or destroy any  improvement
thereon, or any part thereof. Further, Tenant shall not permit any noise or
odor to be emitted  from the  Installations  Premises  which is unlawful or
which constitutes a legal nuisance.  Notwithstanding the foregoing,  Tenant
shall not have any responsibility  for any waste,  noise, or odor caused by
Landlord or its Agents.

                           (iv)   Tenant shall promptly comply in all material
respects  with all present and future laws,  statutes,  ordinances,  rules,
regulations and orders of any federal, state, municipal or other government
or  agency  thereof  having  jurisdiction  over  and  relating  to the use,
condition and occupancy of the Installations  Premises,  and any covenants,
conditions and restrictions of record existing as of the Effective Date and
governing the Installations Premises (collectively, the "Applicable Laws").
Tenant  acknowledges that the Applicable Laws are of public record and that
Tenant knows the character of its operation on the Installations  Premises.
Tenant  shall  have  sole   responsibility  for  its  compliance  with  the
Applicable  Laws in all material  respects,  and  Tenant's  inability to so
comply   shall  not  be  cause  for  Tenant  to   terminate   this   Lease.
Notwithstanding the foregoing:


                                   (A)  Tenant shall not be required to comply
with any  Applicable  Laws to the extent that  Tenant or the  Installations
Premises are legally "grandfathered" or exempt from the application of such
Applicable  Laws or Tenant may obtain from the  appropriate  authorities  a
waiver or variance with respect to compliance.
<PAGE>
 
                                    (B) Tenant shall not be required to comply
with any legal  requirements in connection with the Installations  Premises
arising out of or relating to the  Development or arising out of other acts
or  omissions  of  Landlord  or  its  Agents  on or  with  respect  to  the
Installations  Premises.  All of  such  requirements  on the  Installations
Premises  shall be  promptly  complied  with by  Landlord  in all  material
respects, at its own expense.

                                    (C) Tenant shall not be obligated to
correct any  violations  of  Applicable  Laws which may exist on the
Installations Premises  as of the  Effective  Date,  except  to the  extent
specifically required to do so by written notice from the governmental  
authority having jurisdiction.

                                    (D)  Tenant shall have the right to contest
by appropriate legal proceedings, conducted diligently and in good faith,
without expense to Landlord,  the validity or application of any Applicable
Laws. If compliance  with the Applicable Law being contested may legally be
delayed  pending  the  prosecution  of the  proceeding,  Tenant  may  delay
compliance until the final determination of the proceeding.  Landlord shall
execute and deliver, within ten (10) business days after request by Tenant,
any appropriate  papers and/or other  instruments  that may be necessary to
permit Tenant to contest the validity or application of the Applicable Law.
Landlord shall otherwise  reasonably  cooperate with Tenant in the contest,
provided  that  Landlord  shall not be required to incur any  out-of-pocket
costs  to  third  parties  in  connection  therewith.  Notwithstanding  the
foregoing,  Tenant  shall  ensure that any such  contest does not cause any
lien to be filed against the Installations Premises or any portion thereof.

                  SECTION 6. DEVELOPMENT; DESIGNATION OF THE EXCLUDED
                                     AREAS

                  (a)  Tenant  acknowledges  that,  among  other  purposes,
Landlord purchased the Land for possible  development and/or sale of all or
portions of the  Unimproved  Area.  Although  Landlord  is not  required to
develop or sell any of the Unimproved  Area,  Landlord is  contemplating  a
"mixed-use" development thereon, consisting of, among other things, uses of
varying densities,  sizes and natures, and infrastructure in order to serve
same (all or any portion thereof, the "Development"). The Development shall
be at Landlord's sole cost.  Tenant  acknowledges  that the Development may
require Landlord to change the zoning  classification  of the Installations
Premises. Tenant covenants and  agrees  that  during  the Term it  shall  not
object  to,  and  shall reasonably cooperate with Landlord with respect to,
Development by Landlord or its  successors  of any of the  Unimproved  Area
and  shall  reasonably cooperate so as to avoid interference with the
Development. Notwithstanding the foregoing:
<PAGE>
 
                           (i)  The Development by Landlord shall not
interfere with Tenant's or its subtenants' use of the
Installations Premises.

                           (ii) No rezoning or other development approvals
sought by Landlord for all or any part of the Installations  Premises shall
prohibit,  restrict,  or make  non-conforming  any of the uses which may be
conducted on the Installations  Premises under the I-3 zoning applicable to
such property as of the Effective Date.

                           (iii)  Landlord acknowledges that the
uninterrupted  operation of the  Communications  Facilities  is of critical
importance  to  Tenant.  Landlord  shall not  oppose  the  installation  or
continued operation of the Communications Facilities.

                           (iv)  Tenant shall not be required to incur any
liability  or  out  of  pocket   expense  to  third  parties  in  providing
cooperation to Landlord or its successors with respect to Development.

                  (b)  Intentionally Omitted.

                  (c)  Intentionally Omitted.

                  (d)  In the event  that  ownership  of any  portion of the
Main Building Area or Unimproved  Area is  transferred to another party (an
"Excluded  Area") and such Excluded  Area contains any road,  utility line,
drainage line, storm water management facility,  sign, or other improvement
or  amenity  then  being used by Tenant in  connection  with the  Auxiliary
Buildings or the Communications Facilities (collectively, "Appurtenances"),
then,  automatically  upon such transfer of the Excluded Area, Tenant shall
be deemed to have  easements  for the  continued  uninterrupted  use of the
Appurtenances  for so long as this Lease  remains in effect.  The easements
provided for in this subsection  shall include the right of Tenant to enter
upon the  Excluded  Area to  inspect,  maintain,  repair,  and  replace the
Appurtenances.  Notwithstanding  the  foregoing,  Landlord may  temporarily
discontinue  any  of the  Appurtenances  for a  period  of  not  more  than
forty-eight   (48)  hours  if  Landlord   provides  Tenant  with  temporary
Appurtenances,  provided such temporary  Appurtenances allow Tenant and its
Agents to use the Installations  Premises without any material diminishment
of  Tenant's  or  its  Agents'  use of the  Installations  Premises  or any
material  interference  with  the  business  operations  of  Tenant  or its
subtenants on the Installations Premises.

                  (e)  Notwithstanding anything to the contrary contained in
this  Lease,  Tenant  shall  not be  required  to pay for any  incremental,
additional or increased cost of Insurance or any increased costs of Care of
the  Installations  Premises which are  attributable  to Development or any
other acts or omissions of
<PAGE>
 
Landlord or its Agents with  respect to the  Installations  Premises or the
Unimproved Area.

                  (f)  In   addition   to  the   easements   for   existing
Appurtenances  provided for in  Subsection  6(d),  Landlord  shall grant to
Tenant, or to any utility company or governmental  authority  designated by
Tenant,   such  other   easements  for  utilities,   drainage,   stormwater
management,  and other  similar  matters  on,  under,  and  across the Main
Building  Area  or  Unimproved  Area  as  may  be  reasonably  required  in
connection with the use or occupancy of the  Installations  Premises and in
accordance  with  this  Lease  ("Additional   Easements").   No  Additional
Easements  shall   materially  and  adversely  affect  the  Development  or
materially  increase  the  cost  of  Development.  Landlord  shall  have  a
reasonable  right of approval of the location of all Additional  Easements.
All Additional  Easements  shall be granted  without charge within ten (10)
business  days  after  written  request  by  Tenant,   accompanied  by  the
instrument to be executed and a plat showing the location of the Additional
Easement.  Landlord  shall  make  good  faith and  commercially  reasonable
efforts to cause its mortgagees to subordinate their liens against the Main
Building Area or Unimproved Area to any Additional Easements.

                  SECTION 7.  TAXES AND IMPOSITIONS

                  (a) Tenant  covenants  and agrees to pay,  not later than
the first day on which any  interest or penalty  will accrue or be assessed
for the non-payment  thereof,  all of the following items  applicable to or
affecting  the  Installations  Premises  or any part  thereof  accruing  or
payable from and after the Effective Date and during the Term or applicable
thereto:  (i) all real estate  taxes and  assessments  (including,  without
limitation,  assessments  for special  business  improvement  or assessment
districts),  (ii) personal  property taxes,  (iii) occupancy and rent taxes
specifically  imposed on tenants in Montgomery  County or  attributable  to
subtenants of Tenant,  (iv) water and sewer rents,  rates and charges,  (v)
vault taxes and charges,  (vi) certificate,  license and permit fees, (vii)
any  taxes,  assessments  or  governmental  levies,  general  and  special,
ordinary and extraordinary, foreseen and unforeseen, of any kind and nature
whatsoever  which at any time prior to or during or  applicable to the Term
or any part thereof may be assessed,  levied,  confirmed,  imposed upon, or
grow or accrue or become due and  payable out of, or charged  with  respect
to, or become a lien on, the Installations Premises or any part thereof, or
the  sidewalks  or  streets  in front  of or  adjoining  the  Installations
Premises, or any vault, passageway or space in, over or under such sidewalk
or street, or any other appurtenances to the Installations Premises, or any
personal  property,  equipment  or  other  facility  used in the  operation
thereof,  or any use or occupation of the  Installations  Premises,  or any
document by which Tenant  creates or transfers an interest or estate in the
Installations Premises, and (vii) any fines or penalties or
<PAGE>
 
similar  governmental  charges  applicable  with  respect  to  any  of  the
foregoing,  together  with  interest  and costs  thereon  (all  such  items
aforesaid   may   sometimes   be   collectively   referred   to  herein  as
"Impositions").

                  (b) If, by law, any  Imposition  imposed during the Term,
which is an assessment not related to general real estate taxes, may at the
option of the  taxpayer be paid in  installments  (whether or not  interest
shall accrue on the unpaid balance of such Imposition), Tenant may exercise
the option to pay the same (and any accrued  interest on the unpaid balance
of such  Imposition)  in  installments  and, in such event,  shall pay such
installments plus interest as may become due during the Term of this Lease;
provided, that all such payments shall be made before any fine, penalty, or
other charge for non-payment of any installment may be added thereto.

                  (c) Any Imposition (including,  without limitation, those
Impositions  which have been converted into installment  payments by Tenant
as referred to in Subsection (b) above)  relating to a period of time which
is partially within the Term and partially beyond the Expiration Date shall
be adjusted  between  Landlord and Tenant as of the Expiration Date so that
Landlord shall pay that portion of such Imposition which is attributable to
any  period of time  after the  Expiration  Date and  Tenant  shall pay the
remainder thereof. This subsection shall survive termination of the Lease.

                  (d)  Notwithstanding  the foregoing,  Tenant shall not be
required to pay municipal, state or federal income, excess profits, capital
levy,  rental  (except  as set forth in clause  (iii) of  Subsection  (a)),
estate,  succession,  inheritance,  transfer,  recordation  (except  to the
extent described in Section 30(v)) or gift taxes of Landlord, any corporate
franchise  tax imposed  upon  Landlord  or any tax  imposed  because of the
nature of the business entity of Landlord;  provided,  however,  that if at
any time  during  the  Term,  the  method  of  taxation  prevailing  at the
Effective  Date shall be altered so that,  in  substitution  for ad valorem
real estate taxes, any new Imposition or charge, or any part thereof, shall
be  measured  by or be based in  whole  or in part  upon the  Installations
Premises and shall be imposed upon Landlord,  then all such new Impositions
or charges, or any part thereof,  shall be deemed to be included within the
term  "Impositions",  and Tenant shall pay and discharge the same as herein
provided in respect of the payment of Impositions.

                  (e) (i) If permitted by  Applicable  Law, and provided no
Event of Default is then in existence,  Tenant shall have the right, at its
own expense, to contest the amount or validity, in whole or in part, of any
Imposition by appropriate  proceedings  diligently conducted in good faith.
If, under  Applicable  Law, the  contested  Imposition  must be paid before
undertaking the contest,  Tenant shall pay such  Imposition  (which payment
may be made under protest, at Tenant's option) or, if permitted by
<PAGE>
 
Landlord and any mortgagee of Landlord (including trustees or beneficiaries
of deeds of trust) ("Mortgagee"),  either deposit with Landlord,  Mortgagee
or the  assessing  body, as Landlord,  Mortgagee or the assessing  body may
require,  an amount sufficient to pay all amounts referred to in Subsection
(ii) below,  or  undertake  such other  method of securing  payment of such
amounts as is satisfactory to Landlord, Mortgagee and the assessing body.

                           (ii)    Upon the termination of any such proceeding,
Tenant shall pay the amount of such  Imposition  or part thereof as finally
determined as due in such  proceedings,  the payment of which may have been
deferred  during the  prosecution  of such  proceedings,  together with any
costs,  fees,  interest,  penalties  or  other  liabilities  in  connection
therewith,  and, upon such payment,  Landlord  shall,  provided an Event of
Default is not then in existence, return or request Mortgagee to return any
amount still on deposit with it or with Mortgagee with respect to such paid
Imposition.  If at any time  during  the  continuance  of such  proceedings
Landlord,  Mortgagee or the assessing body shall deem the amount  deposited
or the undertaking insufficient,  Tenant shall, upon thirty (30) days prior
written  notice,  make an additional  undertaking or deposit with Landlord,
Mortgagee or the assessing  body, as Landlord or Mortgagee  reasonably  may
request,  or as the assessing body may require,  and upon failure of Tenant
to do so, the amount  theretofore  deposited  shall be applied by Landlord,
Mortgagee or the  assessing  body to the payment,  removal and discharge of
such Imposition and the interest and penalties in connection  therewith and
any costs, fees (including, without limitation,  reasonable attorneys' fees
and  disbursements) or other liabilities  accruing in any such proceedings,
and the balance, if any, shall be returned to Tenant, or the deficiency, if
any, shall be paid by Tenant immediately on demand of Landlord or Mortgagee
to  the  taxing  authority  to  which  such  Imposition  is  payable.  This
subsection shall survive termination of this Lease.

                  (f)  Landlord  or  Tenant  may,  if it shall  so  desire,
endeavor  at any  time or  times  to  obtain  a  lowering  of the  assessed
valuation upon the  Installations  Premises,  or any part thereof,  for the
purpose of reducing Impositions thereon, and in such event, the other party
will  reasonably  cooperate in effecting such  reduction,  but shall not be
required to incur any out-of-pocket costs in so cooperating.

                  (g)  Landlord  shall  not  be  required  to  join  in any
proceedings  referred to in Subsection  (e) above unless the  provisions of
any law,  rule or  regulation at the time in effect shall require that such
proceedings  be brought by or in the name of  Landlord  or any owner of the
Installations  Premises,  in  which  event,  Landlord  shall  join  in such
proceedings or permit the same to be brought in its name and shall sign all
documents reasonably necessary to prosecute the proceedings. Landlord shall
not be subject to any liability for the payment of any costs or expenses in
connection with any such proceedings, and Tenant will
<PAGE>
 
indemnify  and save  harmless  Landlord from and against any such costs and
expenses,  including,  but not limited to,  reasonable  attorneys' fees and
disbursements, and from any liability resulting from such proceeding. After
reimbursing  and  indemnifying  Landlord  for the items  referred to in the
preceding sentence,  Tenant shall be entitled to any refund with respect to
any  Imposition  and penalties or interest  thereon which have been paid by
Tenant (whether  directly or through  escrowed  funds),  or which have been
paid by Landlord but  previously  reimbursed in full to Landlord by Tenant.
Tenant's right to the refund shall survive termination of this Lease.

                  (h) The certificate,  advice,  statements, or bill of the
appropriate  official  designated  by law to make or  issue  the same or to
receive  payment of any  Imposition  shall be prima facie evidence that the
Imposition  is due and unpaid at the time of the making or issuance of such
certificate, advice statement or bill.

                  (i)  Tenant  shall  make  all  payments  of   Impositions
directly to the appropriate governmental authorities, and Landlord appoints
Tenant  the  attorney-in-fact  of  Landlord  for the  purpose of making all
payments  to be made by Tenant  pursuant to any of the  provisions  of this
Lease to persons or  entities  other than  Landlord.  In case any person or
entity  to whom any sum is  directly  payable  by  Tenant  under any of the
provisions  of this  Section 7 shall  refuse to accept  payment of such sum
from Tenant,  Tenant shall  thereupon  give written  notice of such fact to
Landlord and shall pay such sum  directly to Landlord  and  Landlord  shall
promptly pay such sum to such person or entity.

                  (j) Tenant shall deliver to Landlord  duplicate  receipts
or photostatic copies thereof, or other evidence reasonably satisfactory to
Landlord  (followed by such duplicate  receipts or copies,  when available)
showing the payment of all Impositions, within ten (10) business days after
the  respective  payments  are  required to be made by Tenant and  Landlord
requests such evidence in writing.

                  (k)  Landlord  shall  furnish  to  Tenant  copies  of all
notices of assessment  and bills  relating to  Impositions  within ten (10)
business days after Landlord receives any of the same from the governmental
authorities.

                  SECTION 8.        UTILITIES

                  (a) Tenant  agrees to pay all  charges  made  against the
Installations   Premises  for  gas,  heat,   water,   electricity,   sewage
disposition,  telephone and all other utilities and services related to the
Installations Premises during the Term (collectively, the "Utilities").

                  (b) Tenant understands,  acknowledges and agrees that any
one or more of the Utilities may be  interrupted or diminished by reason of
causes beyond Landlord's reasonable control; that
<PAGE>
 
Landlord does not represent or warrant the  uninterrupted  avail ability of
the Utilities;  that Tenant shall be solely  responsible  for obtaining and
maintaining the use of the Utilities;  and that,  unless caused by Landlord
or its  Agents,  any such  interruption  or  diminishment  shall not (i) be
deemed  an  eviction  or  disturbance  of  Tenant's  right  to  possession,
occupancy and use of the  Installations  Premises or any part  thereof,  or
(ii) render  Landlord  liable to Tenant in damages by  abatement of Rent or
otherwise,  or relieve  Tenant from the obligation to perform its covenants
under this Lease.  In pursuing  the  Development,  Landlord  shall take and
cause  its  Agents  to take  best  efforts  to avoid  the  interruption  or
diminishment of the Utilities.

                  SECTION 9.        SIGNS

                  (a) Tenant shall not  inscribe,  paint,  affix or display
any sign,  advertisement or notice (collectively "Signs") on any portion of
the  Installations  Premises (or on any interior  portion of the  Auxiliary
Buildings that is visible from the exterior) not presently  located thereon
without the prior written  consent of Landlord,  which consent shall not be
unreasonably withheld or delayed.

                  (b) Tenant shall not  inscribe,  paint,  affix or display
any  Sign on any  portion  of the  Unimproved  Area not  presently  located
thereon without the prior written consent of Landlord,  which consent shall
be in Landlord's sole and absolute discretion.
                  (c)  Despite  the  foregoing,  Tenant  shall not  require
Landlord's  consent to replace any Sign existing as of the  Effective  Date
provided that the dimensions and location of the new Sign are substantially
the same as those of the existing Sign.

                  SECTION 10.       AS-IS CONDITION OF INSTALLATIONS
                                    PREMISES

PRIOR TO THE EFFECTIVE DATE, TENANT OWNED THE LAND FOR APPROXIMATELY THIRTY
(30) YEARS,  DEVELOPED THE IMPROVEMENTS THEREON,  INCLUDING BUT NOT LIMITED
TO THE AUXILIARY BUILDINGS AND COMMUNICATIONS  FACILITIES, AND OCCUPIED THE
AUXILIARY  BUILDINGS.  CONSEQUENTLY,  UPON THE EFFECTIVE DATE, TENANT SHALL
ACCEPT THE INSTALLATIONS  PREMISES IN "AS IS" CONDITION.  TENANT AGREES AND
ACKNOWLEDGES  THAT LANDLORD HAS NOT MADE ANY  REPRESENTATION  RESPECTING OR
ANY WARRANTY  WHATSOEVER,  EXPRESS OR IMPLIED,  REGARDING THE INSTALLATIONS
PREMISES,  INCLUDING,  WITHOUT  LIMITATION,  REPRESENTATIONS  OR WARRANTIES
REGARDING THE PHYSICAL NATURE OR CONDITION OF THE  INSTALLATIONS  PREMISES.
TENANT  ACKNOWLEDGES THAT IT IS COGNIZANT OF AND SATISFIED WITH ALL ASPECTS
OF THE INSTALLATIONS PREMISES, AND THAT AS PROVIDED HEREIN THIS TRANSACTION
IS AN "AS IS" TRANSACTION.
<PAGE>
 
                  SECTION 11.       REPAIRS, MAINTENANCE AND MANAGEMENT

                  (a) Tenant shall,  at its sole expense,  keep,  maintain,
manage and operate (collectively,  "Care" or "Care for" or "Care of" as the
context may require) the Installations Premises in a neat, orderly and safe
condition,  in substantially the same manner as Tenant has been maintaining
the Installations Premises as of the Effective Date, and in compliance with
all Applicable Laws in all material respects. In addition, all repairs made
by Tenant shall be reasonably  suited to accomplish their intended purposes
and shall be in compliance with  Applicable Laws in all material  respects.
Tenant  shall in any event make all repairs  necessary  to avoid  damage or
injury to persons or property. Landlord shall have no obligation whatsoever
to Care  for the  Installations  Premises  except  that  Landlord  shall be
responsible  for  any  repairs  necessitated  by  Landlord  or its  Agents.
Further, Tenant, while it was owner of the Property, recorded a Declaration
of Easements,  Covenants,  and Restrictions (the  "Declaration").  Landlord
hereby  delegates to Tenant,  and Tenant hereby accepts from Landlord,  the
maintenance  and  repair  obligations   applicable  to  the  owner  of  the
Installations Premises and described in Section 6 of the Declaration.

                  (b) If any  dispute  shall  arise  between  Landlord  and
Tenant as to the standard of Care required under  Subsection (a) above, the
matter shall be resolved by binding arbitration in accordance with the then
prevailing  Commercial   Arbitration  Rules  of  the  American  Arbitration
Association  (the "AAA").  Upon demand for  arbitration  by either party in
accordance  with  such  rules,  the  matter  shall be  decided  by a single
arbitrator in Montgomery County, Maryland,  selected in accordance with the
prevailing Commercial  Arbitration Rules of the AAA. In deciding any matter
relating to the  Installations  Premises,  the  arbitrator  shall take into
account  the age and  character  of the  improvements  on the  Installation
Premises,  the  standards  of Care set forth in this  Section  11,  and the
remaining Term of this Lease.  The  arbitrator  shall hold a hearing on the
matter within  forty-five (45) days after he or she is appointed.  At least
twenty (20) days before the  hearing,  each party shall submit to the other
party a written  statement of its case,  copies of all documents upon which
it intends to rely at the hearing,  and a list of the  witnesses it intends
to call to testify at the hearing.  The hearing  shall be concluded  within
fifteen (15) days after the initial  hearing date and the arbitrator  shall
decide the matter within  fifteen (15) days after the hearing is concluded.
The arbitrator  shall render his or her decision in writing,  setting forth
the reasons for the  decision.  Each party shall bear its own costs related
to the arbitration,  except that the parties shall share equally all filing
fees and other costs imposed by the AAA in connection  with the arbitration
and the fees of the  arbitrator.  The decision of the  arbitrator  shall be
final and non-appealable and judgment on the decision may be entered in any
court of competent jurisdiction.
<PAGE>
 
                  (c) Unless required by Applicable Laws,  Tenant shall not
be required to (i) improve, upgrade, re-model, retrofit, or renovate all or
any part of the Installations  Premises or (ii) subject to Subsection 11(a)
(excluding the requirement that Care be in substantially the same manner as
Tenant has been maintaining the Installations  Premises as of the Effective
Date),  make any  replacement or major repair of any component,  element or
system of the Installations Premises.

                  (d)  Landlord  agrees that during the Term,  Tenant shall
have the right,  after  providing  prior  written  notice to  Landlord,  to
enforce any rights of Landlord  under  applicable  warranties,  guarantees,
licenses and permits  applicable to the  Installations  Premises.  Landlord
shall sign such documents and otherwise reasonably cooperate with Tenant to
facilitate such  enforcement,  provided that Landlord shall not be required
to incur any out of pocket expense to third parties in so doing.

                  SECTION 12.       ACCESS TO INSTALLATIONS PREMISES

                  Tenant  acknowledges  that  Landlord  has  a  significant
economic  interest in the Care of the safety and  condition  of persons and
property located on and near the Installations  Premises and that Landlord,
pursuant  thereto,   has  a  need,  from  time  to  time,  to  inspect  the
Installations  Premises to insure Tenant's conformity with the covenants of
this Lease. Tenant,  therefore,  hereby authorizes Landlord and any parties
authorized  by Landlord to perform such  inspections  of the  Installations
Premises as Landlord  from time to time may  reasonably  deem  appropriate.
Landlord  shall  have  the  right to  enter  any part of the  Installations
Premises at all reasonable times, upon reasonable  advance notice to Tenant
and accompanied by a representative  of Tenant,  for the purposes of making
such  inspections,   showing  the  Installations  Premises  to  prospective
purchasers,  investors,  mortgagees  and tenants,  and making such repairs,
alterations or improvements to the  Installations  Premises as Landlord may
deem necessary or desirable if Tenant fails to properly Care for same after
notice and  opportunity to cure as provided for in Section  21(a).  No such
entry  shall  materially  interfere  with  the  use  and  occupancy  of the
Installations  Premises by Tenant,  its subtenants and licensees.  Landlord
shall  incur no  liability  to Tenant for such entry  except in the case of
death,  bodily injury, or property damage caused by Landlord or its Agents.
Notwithstanding  the foregoing,  Landlord shall have no right to enter upon
those  portions  of  the  Installations   Premises  which  are  secured  or
confidential  areas  pursuant to  Applicable  Laws or  agreements  to which
Tenant is subject ("Restricted Areas"). Further, no person who is a foreign
national may enter into the Auxiliary Buildings or Antenna Ranges on behalf
of  Landlord,  unless  approval of such  person is obtained  from Tenant in
advance.  Such  approval  may be withheld  only to the extent entry by such
person  would  violate  Applicable  Laws or  agreements  to which Tenant is
subject.

 
<PAGE>
 
                  SECTION 13.       ALTERATIONS AND PERSONAL PROPERTY

                  (a) Tenant,  at its sole  expense and without  Landlord's
approval,  shall  have  the  right  to  make  any  alterations,  additions,
installations or other  improvements  ("Alterations")  to the Installations
Premises,  including,  but  not  limited  to,  constructing  or  installing
additional  Auxiliary  Buildings  and  Communications   Facilities  on  the
Installations  Premises.  In  addition,  Tenant,  at its sole  expense  and
without Landlord's  approval,  may remove or demolish all or any portion of
the  Auxiliary  Buildings  or  Communications   Facilities.  All  buildings
constructed on the Installations Premises after the Effective Date shall be
of a similar or better quality of construction  as the Auxiliary  Buildings
located on the Installations Premises as of the Effective Date.

                  (b) Any  Alteration  shall be  subject,  however,  in all
cases to the following:

                           (i)   All Alterations shall be made at the sole
cost and  expense  of Tenant  and in a good and  workmanlike  manner and in
compliance with all Applicable Laws in all material respects.

                           (ii)  The Installations Premises at all times shall
be kept by Tenant free of liens for labor and materials supplied or claimed
to have been supplied to the Installations  Premises in connection with the
Alterations.

                           (iii) Notice is hereby given that Landlord shall
not be  liable  for any  labor or  materials  furnished  to or for  Tenant.
Furthermore,  notice is  hereby  given to Tenant  and  Tenant's  mechanics,
laborers and materialmen with respect to the Installations Premises that no
mechanic's,  materialman's  or laborer's lien shall attach to or affect the
reversion  or  other  interest  of  Landlord  in  or to  the  Installations
Premises.

                           (iv)  Worker's compensation, builder's risk and
general  liability  insurance with respect to the Alteration as required by
Section 14 shall be maintained by Tenant.

                  (c) All Communication  Facilities and personal  property,
including,  but not limited to,  trade  fixtures,  furniture,  furnishings,
telephone  switching  equipment,  generators and uninterrupted power supply
equipment,   Restricted   Property   and  moveable   equipment   ("Tenant's
Property"), upon or in the Installations Premises shall remain the property
of Tenant or its  Agents,  as the case may be,  or  remain  under  Tenant's
control, and shall be removed by Tenant, or caused to be removed by Tenant,
at its sole cost and expense,  upon  termination of this Lease or surrender
by Tenant of the  Installations  Premises to Landlord;  provided,  however,
that  Tenant  shall not be required  to remove the  footings,  foundations,
concrete bases or underground bases, or underground cabling or conduits and
such items shall become
<PAGE>
 
the property of Landlord upon the expiration or earlier termination of this
Lease. (The term "Tenant's  Property" does not include Fixtures and Limited
Personal Property as defined in the Purchase and Sale Agreement.)  Further,
Tenant  shall  not be  required  to  remove,  or cause to be  removed,  the
property  of  International   Telecommunications   Satellite   Organization
("Intelsat") in the portion of the Installations Premises described in that
certain  Agreement of Lease between  Tenant and Intelsat  dated October 25,
1990. If Tenant's  removal of Tenant's  Property  shall create an unsafe or
unsightly condition, Tenant shall repair the condition at its sole cost and
expense, reasonable wear and tear excepted, but such obligation shall in no
event be deemed to  require  Tenant to remove  the items  which  Tenant may
leave on the Installations Premises pursuant to the preceding provisions of
this  Subsection  (c). If Tenant  fails to so remove and  repair,  Landlord
shall have the right to remove the Tenant's  Property and to dispose of the
same and to repair the  Installations  Premises without  accountability  to
Tenant,  and at the sole  cost  and  expense  of  Tenant.  The  "Restricted
Property" shall mean all personal property,  including,  but not limited to
documents  and  equipment,  which  are  required  to  be  secured  or  kept
confidential  pursuant to Applicable  Laws or agreements to which Tenant is
subject.  The Restricted Property shall at all times, both during and after
the Term, be the sole responsibility of Tenant. Tenant covenants and agrees
that no  Restricted  Property  shall be left in or upon  the  Installations
Premises  after  termination  of this Lease or  surrender  by Tenant of the
Installations  Premises  to  Landlord.   Upon  the  expiration  or  earlier
termination of this Lease all right,  title and interest to any underground
storage tanks located in the  Installations  Premises  shall  automatically
convey to Landlord  without the  requirement  that either party execute any
other  documents  to  effectuate  the  conveyance  and shall become part of
Landlord's property.

                   SECTION 14.      INSURANCE

                  (a) At all times during the Term, Tenant, at its own cost
and  expense,  shall carry and maintain  the  insurance  coverage set forth
below (the "Insurance"):

                  (i)  Hazard insurance covering the Installations
Premises (including,  without limitation,  all Alterations now or hereafter
made to the  Installations  Premises) under an "All Risks of Physical Loss"
policy (an "All  Risks  Policy")  written  in an amount  equal to an amount
necessary  to satisfy  Tenant's  obligations  as lessor,  in the event of a
casualty,  under the Existing  Leases or  Subleases  and the cost of debris
removal and the value of grading,  paving and  landscaping  in the event of
damage caused by casualty to improvements covered by such insurance.

                  (ii) Commercial  liability  insurance with respect to the
Installations Premises and the operations related thereto,
<PAGE>
 
whether conducted on or off the Installations  Premises,  against liability
for death, bodily injury, and property damage (the "Liability Policy"). The
Liability  Policy shall be on an occurrence  basis and  specifically  shall
include:

                        (A) Contractual liability to cover Tenant's
obligations to indemnify Landlord as required under this Lease; and

                        (B) Water damage and sprinkler leakage legal liability.

The  Liability  Policy shall be written for a combined  single limit of not
less than Ten Million Dollars ($10,000,000). Such limit shall be subject to
reasonable  increase  from  time to time  (but not  more  than  once  every
twenty-four   (24)  months)  in  accordance  with  the  limits  then  being
customarily  carried  with  respect to  similar  properties  in  Montgomery
County, Maryland ("Comparable Properties"),  or operations similar to those
being  conducted  on the  Installations  Premises.  Tenant may  satisfy the
required coverage limits for the Liability Policy by carrying a combination
of primary and excess liability policies providing aggregate coverage in at
least the limits stated herein for such policy.

                  (b) Tenant further covenants and agrees, at its sole cost
and expense, to procure and maintain or cause to be procured and maintained
at all times all necessary  worker's  compensation  insurance  covering all
persons  employed by Tenant,  Existing  Tenants and Subtenants in and about
the Installations Premises.

                  (c) Tenant further covenants and agrees, at its sole cost
and expense, to procure and maintain or cause to be procured and maintained
at  all  times  Comprehensive  Automobile  Insurance  covering  all  owned,
non-owned,  and hired  automobiles of tenant in limits of not less than One
Million Dollars ($1,000,000).

                  (d) In  addition  to the  insurance  carried  by  Tenant,
during the course of any Alteration or Care work undertaken by a contractor
hired by or for Tenant,  Tenant  shall  require  such  contractor  to carry
public  liability  insurance in limits of not less than Two Million Dollars
($2,000,000).

                  (e)  Tenant  may  at its  option  provide  any  Insurance
coverage under a blanket  insurance  policy instead of a separate policy or
policies,  provided that the certificate or certificates  issued under such
blanket  insurance policy,  and the coverage afforded thereby,  conforms in
all respects to the requirements hereof.

                  (f) Intentionally Omitted.

                  (g) All  Insurance  shall be in such  form  and  shall be
issued by such responsible insurance companies licensed to do
<PAGE>
 
business in the State of Maryland as are  reasonably  approved by Landlord.
Any insurance  company rated by Bests  Insurance  Reports (or any successor
publication of comparable standing) as A-, VIII or better and by Standard &
Poor's (or any successor of  comparable  standing) as "A" or better (or the
equivalent  of such  rating)  shall be  deemed a  responsible  company  and
acceptable to Landlord.  Upon the Effective Date, and thereafter,  not less
than five (5) days prior to the expiration  dates of the expiring  policies
of Insurance originals of replacement policies or renewal certificates,  as
the case may be,  bearing  notations  evidencing the payment of premiums or
accompanied by other evidence  reasonably  satisfactory to Landlord of each
payment,  shall be delivered by Tenant to Landlord ("Insurance Notice"). If
Tenant does not provide  Landlord  with the  Insurance  Notice or if Tenant
lets any Insurance lapse,  Landlord shall have the right, without providing
any notice to Tenant,  to procure  replacement  coverage  that is effective
upon such  lapse and  Tenant  shall  promptly  reimburse  Landlord  for the
reasonable cost thereof.

                  (h)  All  policies  of  Insurance  shall  name  Landlord,
Tenant,  and,  if  requested  by  Landlord in  writing,  any  Mortgagee  as
insureds,  as their  respective  interests  may  appear.  All  policies  of
Insurance other than the Liability Policy shall, if requested in writing by
Landlord,  name the  Mortgagee  as a loss  payee,  as the  interest of such
Mortgagee  may  appear,  but subject to the  provisions  of Section 15. Any
request of coverage as to a Mortgagee  shall set forth the name and address
of the Mortgagee.

                  (i) Tenant shall not violate or permit to be violated any
of the conditions,  provisions or requirements of any Insurance policy, and
Tenant shall  perform,  satisfy and comply with,  or cause to be performed,
satisfied and complied with, the conditions, provisions and requirements of
all Insurance  policies and the companies writing such policies so that, at
all  times,  the  Insurance  shall  be  provided  by  companies  reasonably
acceptable to Landlord, except as otherwise provided in Subsection 14(g).

                  (j) Each policy or certificate of Insurance shall contain
(i) an  agreement  by the insurer  that such policy  shall not be canceled,
modified or denied renewal  without at least thirty (30) days prior written
notice to  Landlord  and any  Mortgagee  named as an insured or loss payee,
except  that if the  reason  for  cancellation  or  denial  of  renewal  is
nonpayment of premiums, the notice will be at least ten (10) days, and (ii)
a waiver of subrogation by the insurer.

                  (k) If by  reason  of  changed  economic  conditions  the
Insurance  amounts  referred  to in  this  Lease  become  inadequate,  upon
Landlord's request, the limits shall be reasonably increased by Tenant from
time to time (but not more often than once every  twenty-four  (24) months)
to meet  the  changed  conditions,  but any  changes  in  limits  shall  be
consistent with what is customary for
<PAGE>
 
Comparable  Properties or for operations  similar to those conducted on the
Installations  Premises.  Upon the reasonable  request of Landlord,  Tenant
shall  procure  and obtain such types of  insurance  in lieu of the current
insurance  then required under this Lease;  provided that such  replacement
coverage is substantially similar in scope and nature as the Insurance then
required  under this Lease and is the type of  insurance  that is customary
for Comparable  Properties or for operations  similar to those conducted on
the Installations  Premises.  Notwithstanding  the foregoing,  in the event
that  Landlord  makes  such  request  during  the term of the policy of the
insurance that is being  replaced,  Tenant shall not be required to procure
replacement  coverage until the scheduled expiration or earlier termination
of such policy;  provided,  however,  in no event shall such period  exceed
twelve (12) months.

                  (l)  Notwithstanding any provisions in this Section 14 or
elsewhere in this Lease to the  contrary,  any  deductible  to the All Risk
Policy shall be deemed an amount covered by Insurance and shall be promptly
paid by Tenant in the event of damage required to be covered by Insurance.

                  SECTION 15.       DAMAGE OR DESTRUCTION

                  (a)  In  case  of  damage  to  or   destruction   of  the
Installations  Premises  or any  part  thereof  by fire or  other  casualty
("Damage"),  Tenant will promptly give written  notice thereof to Landlord.
In the case of Damage,  whether or not covered by  Insurance,  Tenant shall
continue to pay Rent without any  abatement,  deduction or set-off and this
Lease shall not terminate.

                  (b) All insurance proceeds payable for Damage or for loss
or damage to Tenant's  Property  shall be the exclusive  property of and be
paid  directly  to  Tenant.  Subject to  Subsection  11(a)  (excluding  the
requirement  that Care be in  substantially  the same  manner as Tenant has
been  maintaining  the  Installations  Premises as of the Effective  Date),
Tenant shall have no obligation to restore the Installations Premises after
Damage.

                  (c) Tenant hereby  releases  Landlord and Landlord hereby
releases  Tenant from any and all liability for any loss,  damage or injury
to person or  property  occurring  in,  on,  about or to the  Installations
Properties or personal property by reason of fire or other casualty, to the
extent proceeds of Insurance are received for such loss,  damage, or injury
or to the extent such proceeds  should have been received  under  Insurance
required  to be carried  by this  Lease.  Because  the  provisions  of this
Subsection  will preclude the assignment of any claim  mentioned  herein by
way of  subrogation  or  otherwise  to an  insurance  company  or any other
person,  Tenant shall give to each insurance company which has issued to it
one or more  policies  of  insurance,  notice of the  terms of the  release
contained in this Subsection, and have such
<PAGE>
 
insurance  policies  properly  endorsed,  if  necessary,   to  prevent  the
invalidation of insurance  coverages by reason of the release  contained in
this Subsection.


                  SECTION 16.       INDEMNIFICATION

                  (a) Tenant  will  protect,  indemnify  and save  harmless
Landlord from and against all liabilities,  obligations,  claims,  damages,
penalties,   causes  of  action,  costs  and  expenses  (including  without
limitation,  reasonable  attorneys'  fees  and  expenses)  imposed  upon or
incurred by or asserted  against  Land lord by reason of (i) any  accident,
injury to or death of persons or loss of or damage to property occurring on
or about  the  Installations  Premises  or any part  thereof  caused by the
negligence  or  intentional  misconduct  of Tenant or its Agents;  (ii) any
failure on the part of Tenant to perform or comply with any of the terms of
this Lease; or (iii) performance of any labor or services or the furnishing
of any materials or other property in respect of the Installations Premises
or any part thereof at the  direction  of Tenant.  Tenant  hereby  releases
Landlord from any and all liability for the same.

                  (b) Landlord  will  protect,  indemnify and save harmless
Tenant from and  against all  liabilities,  obligations,  claims,  damages,
penalties,   causes  of  action,  costs  and  expenses  (including  without
limitation,  reasonable  attorneys'  fees  and  expenses)  imposed  upon or
incurred  by or  asserted  against  Tenant by  reason of (i) any  accident,
injury to or death of persons or loss of or damage to property occurring on
or about the  Installations  Premises  or any part  thereof,  caused by the
negligence or  intentional  misconduct of Landlord or its Agents;  (ii) any
failure on the part of  Landlord to perform or comply with any of the terms
of this  Lease;  or  (iii)  performance  of any  labor or  services  or the
furnishing  of  any   materials  or  other   property  in  respect  of  the
Installations  Premises or any part  thereof at the  direction of Landlord.
Landlord hereby releases Tenant from any and all liability for the same.

                  (c) The amount which any party (an "Indemnifying  Party")
is or may be required to pay to any other party (an "Indemnitee")  pursuant
to this  Section  16  shall  be  reduced  (including,  without  limitation,
retroactively)  by any  insurance  proceeds  (or the  amount  of  insurance
proceeds  that would have been  recovered  had  insurance  required by this
Lease been obtained) or other amounts actually recovered by or on behalf of
such  Indemnitee,  in reduction of the related loss. If an Indemnitee shall
have received payment (an "Indemnity  Payment") required by this Lease from
an  Indemnifying  Party  in  respect  of any loss  and  shall  subsequently
actually  receive  insurance  proceeds or other  amounts in respect to such
loss, then such Indemnitee shall pay to such Indemnifying Party a sum equal
to the amount of such insurance proceeds or other amounts actually received
(up to but not in excess of the amount of any Indemnity
<PAGE>
 
Payment made hereunder). An insurer who would otherwise be obligated to pay
any claim shall not be relieved of the responsibility with respect thereto,
or, solely by virtue of the  indemnification  provisions  hereof,  have any
subrogation rights with respect thereto, it being expressly  understood and
agreed  no  insurer  or any  other  third  party  shall  be  entitled  to a
"windfall"  (i.e.,  a benefit  they would not be entitled to receive in the
absence of the indemnification provisions) by virtue of the indemnification
provisions hereof.

                  (d) Procedures for  indemnification of Third Party Claims
shall be as follows:

                           (i)      If an Indemnitee shall receive notice or
otherwise  learn  of  the  assertion  by  a  person   (including,   without
limitation, any governmental entity) who is not a party to this Lease (or a
subsidiary an affiliate of either party) of a claim or of the  commencement
by any such person of any action (a "Third  Party  Claim")  with respect to
which an  Indemnifying  Party may be obligated  to provide  indemnification
pursuant  to this  Section  16 or any other  Section  of this  Lease,  such
Indemnitee  shall  give such  Indemnifying  Party  written  notice  thereof
promptly after becoming aware of such Third Party Claim;  provided that the
failure of any  Indemnitee to give notice as provided in this Section 16(d)
shall not  relieve the  Indemnifying  Party of its  obligations  hereunder,
except to the extent that such Indemnifying Party is actually prejudiced by
such  failure to give notice.  Such notice  shall  describe the Third Party
Claim in reasonable detail.

                           (ii) An  Indemnifying  Party may elect to defend
or to seek to settle or compromise,  at such Indemnifying  Party's own expense
and such  Indemnifying  Party's  own  counsel,  any Third Party  Claim,  as
provided hereafter. Within thirty (30) days after receipt of notice from an
Indemnitee in accordance with Subsection 16(d)(i) (or sooner, if the nature
of such Third Party Claim so requires), the Indemnifying Party shall notify
the Indemnitee of its election whether the  Indemnifying  Party will assume
responsibility  for defending such Third Party Claim.  After notice from an
Indemnifying  Party to an  Indemnitee of its election to assume the defense
of a Third Party Claim, such Indemnifying Party shall not be liable to such
Indemnitee  under this Section 16 for any legal or other  expenses  (except
expenses  approved  in  advance  by the  Indemnifying  Party)  subsequently
incurred  by such  Indemnitee  in  connection  with  the  defense  thereof;
provided that if the defendants  with respect to any such Third Party Claim
include both the Indemnifying  Party and one or more Indemnitees and in any
Indemnitee's reasonable judgment a conflict of interest between one or more
of such Indemnitees and such  Indemnifying  Party exists in respect to such
claim,  such Indemnitees shall have the right to employ separate counsel to
represent  such  Indemnitees  and in that  event  the  reasonable  fees and
expenses of such separate  counsel (but not more than one separate  counsel
reasonably satisfactory to the Indemnifying
<PAGE>
 
Party) shall be paid by such Indemnifying  Party. If an Indemnifying  Party
elects not to assume  responsibility  for defending a Third Party Claim, or
fails  to  notify  an  Indemnitee  of its  election  as  provided  in  this
Subsection  16(d)(ii),  such  Indemnitee  may  defend  or,  subject  to the
remainder of this Subsection  16(d)(ii),  seek to compromise or settle such
Third Party Claim without prejudice to such Indemnitee's rights, if any, to
continue to seek indemnification hereunder.  Notwithstanding the foregoing,
neither an  Indemnifying  Party nor an Indemnitee  may settle or compromise
any claim over the objection of the other; provided,  however, that consent
to settlement or compromise shall not be unreasonably  withheld or delayed.
Neither an Indemnifying  Party nor an Indemnitee  shall consent to entry of
any  judgment or enter into any  settlement  of any Third Party Claim which
does not include as an unconditional  term thereof the giving by a claimant
or plaintiff to such Indemnitee,  in the case of a consent or settlement by
an  Indemnifying  Party,  or to the  Indemnifying  Party,  in the case of a
consent or  settlement  by an  Indemnitee,  of a written  release  from all
liability in respect to such Third Party Claim.

                           (iii) If an Indemnifying Party chooses to defend
or to seek to  compromise  or settle any Third  Party  Claim,  the  related
Indemnitee shall make reasonably  available to such Indemnifying  Party any
personnel or any books,  records or other  documents  within its control or
which it otherwise has the ability to make  available that are necessary or
appropriate for such defense,  settlement or compromise of such Third Party
Claims,   subject   to  the   establishment   of   reasonably   appropriate
confidentiality  arrangements  and  arrangements to preserve any applicable
privilege (including, the attorney-client privilege) and shall cooperate in
such defense, compromise or settlement. If an Indemnifying Party chooses to
defend or to seek to  compromise  or  settle  any Third  Party  Claim,  the
related  Indemnitee shall be entitled to attend and participate in any such
proceeding, discussion or negotiation at its own expense.

                           (iv)  Notwithstanding anything else in this
Section 16 to the contrary,  if an Indemnifying  Party notifies the related
Indemnitee  in writing  of such  Indemnifying  Party's  desire to settle or
compromise  a Third  Party  Claim on the  basis  set  forth in such  notice
(provided that such settlement or compromise  includes as an  unconditional
term thereof the giving by the  claimant or plaintiff of a written  release
of the  Indemnitee  from all  liability  in  respect  thereof  and does not
include any  non-monetary  remedy) and provides the  Indemnitee a copy of a
written  proposal of the applicable  claimant to settle on such terms,  and
the Indemnitee  notifies the Indemnifying  Party in writing within ten (10)
business  days of such notice that such  Indemnitee  declines to accept any
such settlement or compromise, such Indemnitee may continue to contest such
Third Party Claim, free of any participation by such Indemnifying Party, at
such  Indemnitee's  sole  expense.  In such event,  the  obligation of such
Indemnifying Party to such Indemnitee with respect to such Third
<PAGE>
 
Party Claim shall be equal to (i) the costs and expenses of such Indemnitee
prior to the date such  Indemnifying  Party notifies such Indemnitee of the
offer to settle or  compromise  (to the extent such costs and  expenses are
otherwise  indemnifiable  hereunder) plus (ii) the lesser of (A) the amount
of any offer of settlement or compromise which such Indemnitee  declined to
accept and (B) the actual out-of-pocket amount such Indemnitee is obligated
to pay subsequent to such date as a result of such Indemnitee's  continuing
to defend such Third Party Claim (including attorneys fees and expenses).

                           (v)  Any claim on account of a loss which does not
result from a Third Party Claim shall be asserted by written  notice  given
by the  Indemnitee to the related  Indemnifying  Party.  Such  Indemnifying
Party  shall  have a period of 30 days  after the  receipt  of such  notice
within  which to  respond  thereto.  If such  Indemnifying  Party  does not
respond within such thirty (30) day period,  such Indemnifying  Party shall
be deemed to have refused to accept responsibility to make payment. If such
Indemnifying  Party does not respond  within such thirty (30) day period or
rejects such claim in whole or in part,  such  Indemnitee  shall follow the
dispute resolution procedures set forth in Subsection 11(c).

                           (vi) In addition to any adjustments required
pursuant to Subsection  16(c), if the amount of any loss shall, at any time
subsequent to the payment  required by this Lease,  be reduced by recovery,
settlement or otherwise,  the amount of such  reduction,  less any expenses
incurred  in  connection  therewith,   shall  promptly  be  repaid  by  the
Indemnitee to the Indemnifying Party.

                           (vii) In the event of payment by an Indemnifying
Party to any  Indemnitee  in  connection  with any Third Party Claim,  such
Indemnifying  Party shall be subrogated to and shall stand in the place and
the place of such Indemnitee as to any events or  circumstances  in respect
of which such Indemnitee may have any right or claim relating to such Third
Party Claim  against any claimant or plaintiff  asserting  such Third Party
Claim or against any other person.  Such  Indemnitee  shall  cooperate with
such Indemnifying Party in a reasonable manner, and at the cost and expense
of such Indemnifying Party, in prosecuting any subrogated right or claim.

                           (vii) If any indemnity payment required to be made
hereunder is denominated  in a currency  other than United States  dollars,
such payment shall be made in United States  dollars and the amount thereof
shall be  computed  using  the  foreign  exchange  rate  for such  currency
determined  as of the date that  notice of the claim with  respect to which
such indemnity payment is made or given by, or on behalf of, the Indemnitee
to the Indemnifying Party.
<PAGE>
 
                  (e) The  provisions  of this Section 16 shall survive any
termination of this Lease.

                  SECTION 17.       CONDEMNATION

                  (a) If any portion of the Installations Premises is taken
or condemned for a public or quasi-public  use or is sold to a governmental
or  quasi-governmental  authority  in lieu of  condemnation  ("Taken"  or a
"Taking",  as the context shall require),  this Lease shall, as to the part
Taken,  terminate  as of the date that title  shall vest in the  condemning
authority and continue in full force and effect as to the remainder. In the
event of such a  Taking,  Rent  shall  not be  adjusted  and  Tenant  shall
continue to pay Base Rent with respect to the portion of the  Installations
Premises  which has been Taken and Landlord shall retain its remedies under
this Lease with respect to Tenant's obligation to pay such Base Rent.

                  (b) Landlord  and Tenant shall  cooperate in applying for
and  obtaining the maximum  payment or award on account of a Taking.  After
deducting all expenses incurred in connection with obtaining the payment or
award (including reasonable attorneys' fees), the net payment or award (the
"Net Award") shall be distributed  as follows.  Tenant shall be entitled to
claim and receive from the Net Award:  (i) all amounts  designated as being
payable for relocation or similar expenses, (ii) the amount attributable to
the fair market value of Tenant's  leasehold interest in the portion of the
Installations  Premises so Taken, (iii) the amount attributable to the fair
market value of all of Tenant's  Property so Taken or damaged by the Taking
and (iv) the amount  attributable to the fair market value of all buildings
and  improvements  so Taken or damaged by the  Taking,  to the extent  that
Tenant  actually  replaces or repairs same.  Landlord  shall be entitled to
claim and  receive  from the Net Award (i) the amount  attributable  to the
fair market value of Landlord's  fee simple  interest,  as such interest is
subject to this  Lease,  in the  portion of the  Installations  Premises so
Taken,  as such  interest  is  subject to this  Lease,  and (ii) the amount
attributable to the fair market value of Landlord's fee simple interest, as
such  interest  is  subject  to this  Lease,  in all of the  buildings  and
improvements  so Taken or damaged by the Taking,  to the extent that Tenant
does not replace or repair same.  Any dispute  between  Landlord and Tenant
with respect to the distribution  and  apportionment of the Net Award under
the foregoing  provisions  shall be decided by arbitration  pursuant to the
provisions of Subsection  11(b),  with the arbitrator to be a disinterested
appraiser  holding the designation  "MAI" or other equivalent  professional
designation  and  having  at least  ten (10)  years  experience  appraising
commercial  properties in Montgomery County,  Maryland.  Any portion of the
Net  Award  remaining  after  distribution  to  Tenant  and  Landlord,   as
aforesaid, shall belong to Landlord.
<PAGE>
 
                  (c)  In  the  event,   however,  that  the  area  of  the
Installations  Premises shown on Exhibit F or any portion thereof is Taken,
Tenant will not be entitled to make any claim to the  condemning  authority
with respect to that Taking,  except for  relocation  expenses and the fair
market value of Tenant's Property so Taken or damaged by the Taking.

                  (d)  Notwithstanding  the  foregoing  provisions  of this
Section 17, in the case of a Taking of the entire  Installations  Premises,
this Lease  shall  terminate  as of the date that  title  shall vest in the
condemning authority.  Further, in the case that the Taking involves such a
material portion of the Installations  Premises that it is not economically
or technologically  practical for Tenant or Subtenants to use the remaining
portion of the Installations Premises for a satellite earth station, Tenant
may terminate this Lease by written notice to Landlord and this Lease shall
terminate as of the date that title shall vest in the condemning authority.
Upon termination under this Subsection 17(d), Rent shall be adjusted to the
date  of  termination  and  Landlord  and  Tenant  shall  have  no  further
obligations  hereunder,  except with respect to any obligations which arose
or  accrued  before  the  termination  of  this  Lease  or  to  the  extent
obligations  are to survive  the  termination  of this  Lease as  otherwise
expressly  provided in this Lease. Any dispute as to the right of Tenant to
terminate  this  Lease  under  this  Subsection  17(d)  shall be decided by
arbitration in accordance with Subsection 11(b).

                  (e) In no event shall  Tenant be obligated to restore the
Auxiliary  Buildings or any other part of the Installations  Premises which
are damaged by a Taking.  If, however,  this Lease does not terminate under
Subsection  17(d),  Tenant  shall  take such  actions  with  respect to any
damaged portion of the Installations  Premises as are reasonably  necessary
to comply with Tenant's obligations of Care under Subsection 11(a).

                  SECTION 18.       LIENS

                  If,  because of any act or  omission  of Tenant or anyone
claiming by,  through,  or under Tenant,  any mechanic's lien or other lien
shall be filed against the  Installations  Premises or any portion thereof,
or against other property of Landlord, whether or not such lien is valid or
enforceable as such, Tenant shall, at its own expense, cause the same to be
discharged  of record within a reasonable  time,  not to exceed thirty (30)
days, after the date of filing thereof, and shall also defend and indemnify
Landlord and any  Mortgagee and hold them harmless from any and all claims,
losses,  damages,  judgments,  settlements,  costs and expenses,  including
reasonable  attorneys' fees,  resulting therefrom or incurred in connection
therewith.  Tenant shall not  mortgage,  pledge,  hypothecate  or assign as
security its interest in the Installations Premises or under this Lease.
<PAGE>
 
                  SECTION 19.       EXISTING SPACE LEASES; ASSIGNMENT AND
                                    SUBLETTING

                  (a) On the Effective Date, the Installations  Premises is
subject to those  certain  leases  listed in Exhibit G attached  hereto and
made a part hereof (the "Existing Space  Leases"),  which were entered into
by  Tenant,  as  lessor  thereunder,  prior  to  closing  under  the  Sales
Agreement. Landlord has not assumed any of Tenant's obligations thereunder.
For so long as this Lease remains in effect, Tenant covenants and agrees to
continue to discharge  all of the lessor's  obligations  under the Existing
Space Leases, whether accrued or accruing before or during the Term. Tenant
shall indemnify,  defend, and hold harmless Landlord against any obligation
of Landlord under the Existing Space Leases arising or accruing  during the
Term,  except to the extent Tenant is unable to perform such obligations as
a result of Landlord's breach of its obligations  under this Lease.  During
the Term, Tenant shall have the right to (i) collect and retain for its own
account  (subject to Subsection (c) below) all rents and other payments due
from tenants under Existing Space Leases  ("Existing  Tenants"),  (ii) hold
and apply any security  deposits of Existing Tenants in accordance with the
Existing Space Leases  (provided,  however,  that Tenant shall deliver such
security  deposits to Landlord at the expiration or earlier  termination of
this Lease to the extent such  security  deposits  have not been applied by
Tenant  pursuant to the applicable  Existing Space Lease or returned to the
applicable  Existing Tenant, and the term of the applicable  Existing Space
Lease extends beyond the Term),  and (iii)  terminate,  modify or otherwise
deal with the  Existing  Space  Leases as Tenant,  in its sole  discretion,
deems appropriate;  provided,  however,  (A) such right shall automatically
end upon the  termination  of this  Lease for any  reason,  (B) none of the
Existing Space Leases shall be modified to provide for a term which extends
beyond the Term,  unless  Landlord gives its prior written  consent to such
extended term,  which consent may be granted or withheld in Landlord's sole
discretion,  (C) each  modification  of an  Existing  Space  Lease shall be
subject and  subordinate to this Lease,  and in the event of the expiration
or termination  of this Lease,  Landlord shall not be required to recognize
such  modification  without its express  written  agreement  to be bound by
same,  which may be given or not in  Landlord's  sole  discretion,  and (D)
Tenant  shall  promptly   provide  to  Landlord  copies  of  all  documents
terminating or modifying any Existing Space Lease.

                  (b)  Tenant may  assign  its  interest  in this Lease (an
"Assignment") or sublet all or any portion of the Installations Premises (a
"Sublease"), without the consent of Landlord. The term of any such Sublease
shall not exceed the Term,  unless Landlord gives its prior written consent
to such  extended  term,  which  consent  may be  granted  or  withheld  in
Landlord's  sole  discretion.  In the event of any  Assignment or Sublease,
Tenant shall  nevertheless at all times remain fully responsible and liable
for the payment of Rent and the performance and observance
<PAGE>
 
of all of Tenant's other  obligations under this Lease. Each Sublease shall
be subject and subordinate to this Lease. If requested by Tenant,  Landlord
shall enter into a nondisturbance agreement with respect to any Sublease on
substantially  the  same  terms  as are  contained  in  the  nondisturbance
agreement  attached  hereto as Exhibit H (the  "Landlord  Approved  SNDA"),
provided  that (i) the Sublease does not, in any material  respect,  impose
greater  obligations  on the  sublandlord  or grant  greater  rights to the
subtenant  than those  existing as of the Effective Date under the Existing
Space  Leases,  (ii) term of the  Sublease  does not  extend  past the Term
(unless  expressly  approved  by  Landlord  as  aforesaid),  and  (iii) the
Sublease (y) is an arms-length transaction and such subtenant was obtained,
and the economic  terms of such Sublease were  negotiated by, a third party
leasing  agent using  marketing  efforts  customarily  used for  Comparable
Properties,  or (z)  requires  the  subtenant to pay basic rent at the then
current  Base Rent rate per  square  foot  under  this Lease and a pro rata
share of  increases  in  operating  expenses,  Impositions  and  insurance.
Landlord  agrees at any time  hereafter,  upon ten (10) business days prior
written  notice,  to  execute  and  deliver,  and  cause its  Mortgagee  to
recognize,  the Landlord Approved SNDA, provided the Landlord Approved SNDA
has been duly executed by Tenant and such subtenant. Promptly upon entering
into any Assignment or Sublease,  Tenant shall provide Landlord with copies
of all documents effecting such Assignment or Sublease.

                  (c) If there is an Event of  Default,  in addition to any
other  remedies  provided  by this  Lease  or by law or in  equity,  at its
option, Landlord may collect directly from any Existing Tenant, Assignee or
lessee under a Sublease  ("Subtenant")  all rent  becoming due to Tenant by
reason of the  applicable  Existing  Lease,  Assignment  or  Sublease.  Any
collection  by Landlord  from the  Existing  Tenant,  Assignee or Subtenant
shall not be construed  to  constitute a novation or release of Tenant from
the  further  performance  of  its  obligations  under  this  Lease  or  an
acceptance of the terms of such Existing Lease, Assignment or Sublease.

                  (d) Any amounts collected by Landlord pursuant to Section
19(c) or otherwise from an Existing Tenant, Assignee or Subtenant which are
applicable to any period  preceding the Expiration Date (had this Lease run
its full  Initial  Term as may have been  extended),  shall be  applied  by
Landlord to the reduction of Tenant's  remaining  liability,  if any, under
this Lease.

                  SECTION 20.       SUBORDINATION OR SUPERIORITY OF LEASE

                  (a) Except as  otherwise  provided in this  Section,  the
rights  and  interest  of Tenant  under this  Lease  shall be  subject  and
subordinate  to any  mortgages  that may be placed  upon the  Installations
Premises  and to any and all  advances  to be made  thereunder,  and to the
interest thereon, and all renewals, replacements,  extensions, bifurcations
and splits thereof,  if the Mortgagee named in said mortgage shall elect to
subject and
<PAGE>
 
subordinate  the rights and interest of Tenant under this Lease to the lien
of its mortgage. Any Mortgagee may elect to give the rights and interest of
Tenant  under this Lease  priority  over the lien of its  mortgage.  In the
event of either such election and upon  notification  by such  Mortgagee to
Tenant to that  effect,  the rights and interest of Tenant under this Lease
shall be deemed to be subordinate to, or to have priority over, as the case
may be, the lien of said mortgage,  whether this Lease is dated prior to or
subsequent to the date of said mortgage without any further action required
by Landlord or Mortgagee;  provided, however, that as a condition precedent
to any  subordination  of  this  Lease,  Landlord  delivers  in  advance  a
non-disturbance  agreement  duly executed by Mortgagee  and  Landlord.  The
non-disturbance  agreement shall be  substantially  in the form attached to
and made a part of this Lease as Exhibit I (the "Tenant  Approved SNDA") or
in a  commercially  reasonable  form and subject to the approval of Tenant,
not to be unreasonably withheld.  Tenant agrees at any time hereafter, upon
ten (10)  business days prior  written  notice,  to execute and deliver the
Tenant Approved SNDA.

                  (b)  Nothing  contained  in this  Lease  shall  limit  or
curtail  Landlord's right to sell,  mortgage or otherwise  transfer its fee
interest  in the  Installations  Premises,  or affect  Landlord's  right to
assign the Rent  payable  under this Lease  either as  collateral  security
under a  mortgage  or  otherwise.  Any such  sale,  mortgage,  transfer  or
assignment shall be binding on Tenant but shall be subject to this Lease.

                  (c) Landlord  acknowledges  that the term of the Intelsat
Lease extends beyond the Expiration Date.

                  SECTION 21.       DEFAULTS AND REMEDIES

                  (a) The  occurrence  of any one or more of the  following
events shall be a default and breach of this Lease by Tenant (collectively,
"Events of Default"):

                           (i)      Tenant shall fail to pay any installment of
Rent when the same shall be due and payable  and fail to cure such  default
within  nine  (9)  days  after  receiving   written  notice  from  Landlord
specifying  the default.  For the purposes of this  Subparagraph  21(a)(i),
Tenant  agrees notice by Mortgagee,  or Landlord's  loan service  provider,
given  according  to the terms of Section  26,  shall be deemed  sufficient
notice.

                           (ii)     Tenant shall fail to perform or observe any
other term,  condition,  covenant or obligation required to be performed or
observed  by it under  this  Lease for a period of thirty  (30) days  after
written notice from Landlord  specifying such default;  provided,  however,
that if the term,  condition,  covenant or  obligation  to be  performed by
Tenant is of such  nature  that the same  cannot  reasonably  be  performed
within such thirty (30) day period,  such  default  shall be deemed to have
been cured if Tenant  commences such  performance  within the 30 day period
and thereafter
<PAGE>
 
diligently  undertakes  to  complete  the cure and in all events  cures the
default within one hundred twenty (120) days of Landlord's notice,  subject
to extension for Unavoidable Delays.

                           (iii) An Event of Default,  as defined  therein,
shall occur under that certain lease agreement, dated of even date herewith,
between Landlord and Tenant with respect to the Master Lease Property (the
"Master Lease").

                           (iv)  Termination or rejection of this Lease
pursuant to Section 22.

                           (v)   Assumption or assignment of this Lease, under
the  conditions  referred  to in Section  22,  unless the  requirements  of
Section 22 applicable to such assumption or assignment are satisfied.

                  (b) Upon the occurrence of any Event of Default, Landlord
shall have the following rights and remedies,  in addition to those allowed
by law or equity,  any one or more of which may be  exercised  concurrently
and without further notice to or demand upon Tenant:

                           (i)      Landlord may re-enter the Installations
Premises  and cure any  default  of Tenant,  in which  event  Tenant  shall
reimburse Landlord as Additional Rent for any reasonable costs and expenses
which  Landlord may incur to cure such default;  and Landlord  shall not be
liable to Tenant for any loss or damage  which Tenant may sustain by reason
of Landlord's action, except loss or damage caused by Landlord's negligence
or intentional misconduct.

                           (ii)     Landlord may terminate this Lease, in which
event:  (A) Tenant shall not  thereafter  be entitled to  possession of the
Installations  Premises and Tenant shall immediately  thereafter surrender,
or cause to be surrendered,  the  Installations  Premises to Landlord;  (B)
Landlord may re-enter the  Installations  Premises and dispossess Tenant by
summary  proceedings,  ejectment or other legal  process and may remove its
effects,  without prejudice to any other remedy which Landlord may have for
possession or  arrearages  in Rent;  and (C) Tenant shall be liable for all
loss or damage which Landlord may sustain by reason of such termination and
re-entry;  and  Landlord  may re-let  all or any part of the  Installations
Premises  for a  term  different  from  that  which  would  otherwise  have
constituted  the  balance  of the  Term  and  for  Rent  and on  terms  and
conditions different from those contained herein, whereupon Tenant shall be
obligated  to pay to  Landlord  the  deficiency,  if any,  between the Rent
provided  for  herein  and  that  provided  for in  any  lease  covering  a
subsequent re-letting of the Installations  Premises,  for the period which
would otherwise have constituted the balance of the Term, together with all
of Landlord's  reasonable costs and expenses of preparing the Installations
Premises  for  re-  letting,   including   all   repairs,   tenant   finish
improvements,  brokers' and  attorneys'  fees, and all loss or damage which
Landlord
<PAGE>
 
may sustain by reason of such re-letting, it being expressly understood and
agreed that the liabilities and remedies  specified above shall survive the
termination of this Lease. Landlord shall make diligent efforts to mitigate
its damages in the event of an Event of Default.

                           (iii) (A) Notwithstanding the termination of this
Lease,  Landlord  may declare all Base Rent which would have been due under
this Lease for the balance of the Term to be  immediately  due and payable.
In that  event,  Tenant  shall be  obligated  to pay an amount in cash that
would be necessary to purchase U.S.  Obligations in such amounts and having
such  maturities  that the principal and interest of such U.S.  Obligations
would be  sufficient  to provide funds as close as possible but in no event
less or later  than the  payments  due under this Lease as Base Rent as and
when such  payments  would be due if no  acceleration  of the Base Rent had
occurred (the "Accelerated  Payment").  U.S. Obligations are obligations or
securities not subject to prepayment,  call or early  redemption  which are
direct obligations of, or obligations fully guaranteed as to timely payment
by, the United States of America or any agency or instrumentality  thereof,
the  obligations  of which are  backed by the full  faith and credit of the
United States of America.

                           (B) Upon receipt of the Accelerated Payment,
Landlord shall use commercially reasonable efforts to lease the portions of
the  Installations  Premises  not  subject  of  other  leases  at the  then
currently  existing fair market rate for  Comparable  Properties.  From and
after the Event of Default all rent and other payments, except for security
deposits,  collected  under leases  entered into after the Event of Default
("Post Default Leases") and under any Existing Space Leases,  Subleases, or
occupancy  or  concession  agreements  in effect as of the Event of Default
shall be placed in a separate escrow account (the "Escrow Account") held by
a mutually  acceptable  escrowee  (the  "Escrowee").  Also,  any refunds of
Impositions  paid by Tenant and received by Landlord shall be placed in the
Escrow  Account.  The Escrow Account shall be interest  bearing and earn at
least a money  market rate of interest.  All  interest  earned shall become
part of the escrow fund and be treated in the same manner as the  principal
in the Escrow  Account.  The Post  Default  Leases and all  Existing  Space
Leases,  Subleases, and occupancy and concession agreements in effect as of
the  Event of  Default  are  collectively  referred  to as the  "Mitigation
Leases".  The funds  contained in the Escrow  Account shall be disbursed in
the following  manner and with the following  priority:  (i) all direct and
reasonable  operating  expenses  ("Operating  Expenses")  paid by  Landlord
relating  to the  Installations  Premises  during  the  "Default  Measuring
Period"  (defined  below) shall be paid to Landlord,  (ii) a management fee
equal to three  percent (3%) of the amount  collected by Landlord  from the
Mitigation  Leases  during the Default  Measuring  Period  shall be paid to
Landlord,  (iii) all  direct and  reasonable  costs of  collection  paid by
Landlord to collect the sums payable under the  Mitigation  Leases shall be
paid to Landlord,  (iv) reasonable and customary brokerage fees, reasonable
attorneys' fees
<PAGE>
 
and all other  reasonable  out of pocket costs paid by Landlord  during the
Default Measuring Period to third parties in connection with the Mitigation
Leases (including the fees of the Escrowee and the Escrow Account) shall be
paid to Landlord, (v) Tenant shall be paid up to the amount that Tenant was
scheduled to pay as Base Rent during the Default Measuring Period, and (vi)
the remainder  shall be shared  equally by Landlord and Tenant.  As used in
this Section,  the term "Default Measuring Period" shall mean (1) as to the
payments  contemplated in (i) through (iv) above,  each consecutive one (1)
month period after the  establishment  of the Escrow Account until the date
the Term would have  expired but for the early  termination  of this Lease,
and  (2) as to the  payments  contemplated  in (v)  and  (vi)  above,  each
consecutive  three (3) month period after the  establishment  of the Escrow
Account  until  the date the Term  would  have  expired  but for the  early
termination of this Lease.  The first Default  Measuring  Period,  however,
shall also  include the period from the date of the Event of Default to the
date the Escrow Account was established.  Escrowee shall make disbursements
to the parties in the priority  described  above on or about the  fifteenth
(15th) day after the end of each Default Measuring Period.  Within ten (10)
days  after  the end of  each  Default  Measuring  Period  relating  to the
payments  contemplated in (i) through (iv) above, Landlord shall furnish to
Escrowee and Tenant a written  statement setting forth in reasonable detail
all amounts paid into the Escrow Account and all amounts for which Landlord
is claiming  payment  with  respect to the  immediately  preceding  Default
Measuring  Period.  The  statement  shall be certified by Landlord as being
accurate  and shall be  accompanied  by  invoices  and other  documentation
reasonably  evidencing  the amounts  claimed for  payment.  Escrowee  shall
withhold from the payments made under (v) and (vi) above  one-fourth of the
reasonably  estimated  real estate taxes and Insurance  premiums  which are
next due  respect to the  Installations  Premises,  less the amounts of any
real estate taxes or  Insurance  premiums  required to be directly  paid by
tenants under the  Mitigation  Leases (the "Tax and Insurance  Holdbacks").
Escrowee shall release the Tax and Insurance Holdbacks to Landlord when and
to the extent that real estate taxes and Insurance  premiums are payable by
Landlord with respect to the  Installations  Premises.  At the time of each
such  disbursement to Landlord,  Escrowee shall also disburse to Tenant the
amount,  if any, by which the Tax and Insurance  Holdbacks held by Escrowee
exceed the tax bill or Insurance  premiums paid with such Tax and Insurance
Holdbacks. Although the last Default Measuring Period shall end on the date
the Initial Term would have expired but for the early  termination  of this
Lease, any amounts  collected by Landlord under the Mitigation Leases after
that date and  attributable to any period which occurred prior to that date
shall be paid  directly by  Landlord to Tenant.  No party shall be paid for
any amount  described in this  Subsection  (B) to the extent such party was
otherwise  reimbursed  for  such  amount,  including,  without  limitation,
previous  payment  by the  Escrowee  or  previous  reimbursement  under the
Mitigation Leases. For the purposes of this Section,  "Operating  Expenses"
excludes  (1) debt  service and other costs of  Landlord's  financing,  (2)
capital expenditures,
<PAGE>
 
except  to  the  extent  that   Landlord  is   obligated  to  make  capital
improvements  under the terms of Existing  Space  Leases and  Subleases  in
effect as of the Event of  Default,  and then only to the  extent  that the
amortized cost of such capital  improvements  is properly  allocable to the
Default   Measuring  Period,   (3)  management  fees,  (4)  overhead,   (5)
depreciation,  (6) accountants' fees, (7) any losses or expenses covered by
Insurance,  whether  or  not  such  Insurance  is in  fact  maintained,  or
compensable by condemnation  proceeds, (8) any amount incurred by reason of
the  negligence or  intentional  misconduct of Landlord or its Agents,  (9)
fines or  penalties,  and (10) any amounts  paid by specific  tenants.  Any
refunds,  discounts,  or  recoupments  of  Operating  Expenses  received by
Landlord  shall be accounted  for by Landlord and credited  back to Tenant.
Tenant, at reasonable times and upon reasonable notice shall have the right
to audit  Landlord's  books and  records  relating to  Operating  Expenses;
provided,  however,  in no event  shall  Landlord  be required to keep such
underlying receipts for Operating Expenses beyond a date which is three (3)
years after Escrowee makes a disbursement for such Operating Expenses.  The
cost of capital expenditures which may be included as Operating Expenses as
provided above shall be amortized over the useful life of each such capital
expenditure as determined for federal income tax purposes.  Notwithstanding
the  foregoing,  Landlord  shall not be entitled to payment from the Escrow
Account for any costs of repairs or maintenance  that Tenant would not have
been obligated to undertake under the standard of Care set forth in Section
11 of this Lease or for any other  Operating  Costs which  Tenant would not
have been  obligated to pay under this lease.  Notwithstanding  anything to
the contrary  contained  herein,  until the expiration of 91 days after the
receipt of the Accelerated Payment, Tenant shall not make any claim against
Landlord that  Landlord has failed to use diligent  efforts to mitigate its
damages  after  the Event of  Default  which  gave rise to the  Accelerated
Payment. This provision shall survive the termination of this Lease.

                           (iv)   Landlord may sue for injunctive relief or to
recover damages for any loss resulting from the breach.

                  (c) Any agreement for an extension of the Term or for any
other  additional  period after the Term shall not thereby prevent Landlord
from  terminating this Lease for any reason specified in this Lease. If any
such right of termination  is exercised by Landlord  during the Term or any
extension  thereof,  Tenant's  right to any extension or additional  period
shall thereby be automatically  canceled.  Any such right of termination of
Landlord contained herein shall continue during the Term and any subsequent
extension hereof.

                  (d) The  failure  or delay by either  party to enforce or
exercise at any time any of the rights or remedies or other  provisions  of
this Lease shall not be  construed to be a waiver  thereof,  nor affect the
validity of any part of this Lease or the right of that party thereafter to
enforce each and every such right or remedy or other provisions.  No waiver
of any default or breach
<PAGE>
 
of this Lease shall be held to be a waiver of any other  default or breach.
No act or  omission  by  Landlord  during  the  Term  shall  be  deemed  an
acceptance of a surrender of the Installations  Premisesand no agreement to
accept  such a  surrender  shall be valid  unless in writing  and signed by
Landlord.

                  (e) If either  party  defaults  under  this Lease and the
other party places the  enforcement of all or any part of this Lease or the
collection of any sum due or to become due under this Lease or the recovery
of  possession of the  Installations  Premises in the hands of an attorney,
and such party prevails in litigation concerning such issue, the defaulting
party  agrees  to  reimburse  the  prevailing   party  for  the  reasonable
attorney's fees incurred thereby.

                  SECTION 22.       BANKRUPTCY OR INSOLVENCY

                  Landlord and Tenant agree that the following  shall apply
in the event of the bankruptcy or insolvency of Tenant:

                  (a) If a petition  is filed by, or an order for relief is
entered  against  Tenant  under  Chapter 7 of the  Bankruptcy  Code and the
trustee of Tenant  elects to assume this Lease for the purpose of assigning
it, such assumption and assignment may be made only if all of the terms and
conditions of Subsections (b) and (d) below are satisfied. To be effective,
an election to assume this Lease must be in writing, addressed to Landlord,
and  all of  the  conditions  herein  stated,  which  Landlord  and  Tenant
acknowledge to be commercially reasonable, must have been satisfied. If the
trustee  fails so to elect to assume  this Lease  within 60 days after such
filing or order or such additional time as the Bankruptcy Court, for cause,
may fix,  this Lease  will be deemed to have been  rejected,  and  Landlord
shall then  immediately  be entitled  to  possession  of the  Installations
Premises  without  further  obligation  to Tenant or the trustee,  and this
Lease shall be terminated.  Landlord's  right to be compensated for damages
in the bankruptcy proceeding, however, shall survive such termination.

                  (b) If Tenant files a petition for  reorganization  under
Chapters 11 or 13 of the  Bankruptcy  Code, or if a proceeding  filed by or
against Tenant under any other chapter of the Bankruptcy  Code is converted
to a  chapter  11 or 13  proceeding  and  Tenant's  trustee  or  Tenant  as
debtor-in-possession  fails to assume  this  Lease  within 60 days from the
date of the filing of such petition or conversion or such  additional  time
as the  Bankruptcy  Court,  for  cause,  may fix,  then the  trustee or the
debtor-in-possession  shall be deemed to have  rejected  this Lease.  To be
effective, any election to assume this Lease must be in writing,  addressed
to Landlord  and, if there has been a default  under the Lease,  all of the
following   conditions,   which  Landlord  and  Tenant  acknowledge  to  be
commercially reasonable, must have been satisfied:
<PAGE>
 
                           (i)      The trustee or the debtor-in-possession has
                                    cured or has provided to Landlord adequate
                                    assurance that:

                                    (A)     It  will   cure  all   monetary
                                            defaults   under   this   Lease
                                            within   the   number  of  days
                                            specified  in Section  21(a)(I)
                                            of this  Lease from the date of
                                            assumption; and

                                    (B)     It will  cure  all  nonmonetary
                                            defaults   under   this   Lease
                                            within   the   number  of  days
                                            specified in Section  21(a)(ii)
                                            of this  Lease from the date of
                                            assumption.

                           (ii)     The trustee or the debtor-in-possession
                                    has   compensated   Landlord,   or  has
                                    provided    Landlord    with   adequate
                                    assurance   that   Landlord   will   be
                                    compensated  promptly for any pecuniary
                                    loss it has  incurred  arising from the
                                    default of Tenant, the trustee,  or the
                                    debtor-in- possession.

                         (iii)      The trustee or the debtor-in-possession
                                    has  provided  Landlord  with  adequate
                                    assurance of the future  performance of
                                    each of Tenant's obligations under this
                                    Lease; provided,
                                    however, that:

                                    (A)     From  and  after  the  date  of
                                            assumption of this Lease, until
                                            the date of the  assignment  of
                                            this  Lease,  it shall  pay all
                                            monetary           obligations,
                                            including,  without limitation,
                                            the  Rent  payable  under  this
                                            Lease,  in advance on each date
                                            that such amounts are payable.

                                    (B)     It  shall  also   deposit  with
                                            Landlord,  as security  for the
                                            timely   payment  of  Rent,  an
                                            amount  equal to three  months'
                                            Base  Rent and  other  monetary
                                            charges   accruing  under  this
                                            Lease;

                                    (C)     If not  otherwise  required  by
                                            the  terms  of this  Lease,  it
                                            shall also pay in  advance,  on
                                            each day  that any  installment
                                            of Base Rent is  payable,  one-
                                            twelfth of Tenant's Imposition,
                                            Insurance and other obligations
                                            under this Lease.

                  (c)  If  the  trustee  or  the  debtor-in-possession  has
assumed this Lease,  pursuant to Subsection (a) or (b) above, and elects to
assign  Tenant's  interest  under this Lease or the estate  created by that
interest to any other person,  such interest or estate may be assigned only
if the intended assignee has provided


 
<PAGE>
 
adequate assurance of future performance of all of the terms,
covenants, and conditions of this Lease.

For the  purposes of this  Subsection  (d),  "adequate  assurance of future
performance" means that Landlord has ascertained that each of the following
condition has been satisfied:

The assignee has submitted a current financial  statement which shows a net
worth and  working  capital  in  amounts  sufficient  to assure  the future
performance by the assignee of Tenant's obligations under this Lease.

                  (d) When, pursuant to the Bankruptcy Code, the trustee or
the  debtor-in-possession  is obligated to pay reasonable use and occupancy
charges  for the use of all or part of the  Installations  Premises,  it is
agreed that such  charges will not be less than the Base Rent as defined in
this Lease,  plus Additional Rent and other monetary  obligations of Tenant
included herein.

                  (e)  Except  to  the  extent  provided  by  law,  neither
Tenant's  interest  in this Lease nor any estate of Tenant  created in this
Lease  shall pass to any  trustee,  receiver,  assignee  for the benefit of
creditors, or any other person or entity, nor otherwise by operation of law
under the laws of any state having  jurisdiction  of the person or property
of Tenant, unless Landlord consents in writing to such transfer. Landlord's
acceptance  of Rent or any  other  payments  from  any  trustee,  receiver,
assignee,  person,  or other entity will not be deemed to have  waived,  or
waive,  either the requirement of Landlord's consent or Landlord's right to
terminate this Lease for any transfer of Tenant's interest under this Lease
without such consent.

                  SECTION 23.       SURRENDER OF INSTALLATIONS PREMISES

                  Upon the expiration or earlier termination of this Lease,
Tenant shall surrender the Installations Premises to Landlord, broom-clean,
in good order,  condition and repair (except for ordinary wear and tear and
conditions  which  existed  on  the  Installations  Premises  prior  to the
Effective  Date and subject to Sections  15 and 17),  free of all  Tenant's
Property  (except as  otherwise  provided  in  Subsection  13(c),  Personal
Property and Restricted  Property (except for Restricted Property belonging
to any  Existing  Tenant or  Subtenant  whose  Existing  Lease or  Sublease
Landlord  has  expressly  agreed  in  writing  may  continue),  and free of
violation of Applicable  Laws in all material  respects  subject to Section
5(b)(iv).  To the extent  Tenant fails to comply with the  requirements  of
this  Section,  Landlord  may  restore the  Installations  Premises to such
condition at Tenant's expense.

                  SECTION 24.       NON-CONSENSUAL HOLDING OVER

                  In  the  event  Tenant   remains  in  possession  of  the
Installations  Premises or any part thereof without the consent of Landlord
after the expiration or earlier termination of this Lease,
<PAGE>
 
Tenant shall be deemed, at Landlord's  election,  to hold the Installations
Premises as a tenant at sufferance subject to all of the terms, conditions,
covenants and  provisions  of this Lease which shall be  applicable  during
such time (the "Holdover  Period"),  except that, for each month during the
Holdover  Period,  Tenant shall pay to Landlord 150% of the sum of the last
current full monthly  installment of Base Rent plus Additional  Rent, which
shall be payable to Landlord  within five (5) business  days of notice from
Landlord.  In addition,  such  election  shall not preclude  Landlord  from
seeking, and shall be cumulative with, any other remedy under this Lease or
granted by law or in equity.  No holding  over by Tenant,  as  described in
this Section 24,  whether  with or without the consent of  Landlord,  shall
operate to extend this Lease.

                  SECTION 25.       QUIET ENJOYMENT

                  If and for so long as Tenant pays the prescribed Rent and
performs  or  observes  all  of  the  terms,   conditions,   covenants  and
obligations  of this Lease  required  to be  performed  or  observed  by it
hereunder, Tenant shall at all times during the Term have the peaceable and
quiet  enjoyment,  possession,  occupancy  and  use  of  the  Installations
Premises without any interference from Landlord, or anyone claiming through
or under  Landlord,  subject to any  matters of record as of the  Effective
Date to which this Lease is subject.

                  SECTION 26.       NOTICES

                  Any notice, demand or request required or permitted to be
given  under  this Lease or by law shall be deemed to have been given if in
writing and delivered addressed to the party who is to receive such notice,
demand or request at the address  set forth below or at such other  address
as  Landlord or Tenant may  specify  from time to time by notice.  Delivery
hereunder  shall be deemed  to  include  pre-paid  courier  delivery  (by a
reputable  courier delivery  service),  pre-paid  overnight  delivery (by a
reputable  overnight delivery  service),  postage and fees paid U.S. Postal
Service  express mail or  certified  mail,  return  receipt  requested,  or
facsimile transmission with electronic  verification during normal business
hours,  if sent to the address of the  parties  designated  hereunder,  and
shall be deemed  received on the next business day such notice is delivered
or  refused  at such  address  except  notices  sent by fax shall be deemed
received upon electronic verification.

         Landlord:        LCOR Incorporated
                          6701 Democracy Boulevard
                          Bethesda, MD  20817
                          Attn: Mr. R. William Hard
                          Facsimile: (301) 897-3713

         with copies to:  Jones, Day, Reavis & Pogue
                          1450 G Street, N.W.
                          Washington, DC 20005-2088
                          Attn: Sigmund T. Weiner, Esq.
                          Facsimile: (202) 737-2832
<PAGE>
 
                          LCOR Incorporated
                          300 Berwyn Park, Suite 104
                          Berwyn, PA 19312
                          Attention: Mr. Peter DiLullo
                          Facsimile: (610) 408-4420

         Tenant:          COMSAT Corporation
                          6560 Rock Spring Drive
                          Bethesda, MD 20817
                          Attention: General Manager Corporate
                              Services
                          Facsimile: (301) 214-7147

         with a copy to:  COMSAT Corporation
                          6560 Rock Spring Drive
                          Bethesda, MD 20817
                          Attention: General Counsel

                          Facsimile: (301) 214-7128

Any copies required to be sent as above provided are for the convenience of
the  parties  and no such copy  shall  constitute  adequate  notice for the
purposes of this Section.

                  SECTION 27.       HAZARDOUS MATERIALS

                  Landlord  and Tenant agree as follows with respect to the
existence or use of "Hazardous Material" on the Installations Premises.

                  (a)  If  the  use,  storage,  handling,   generation,  or
disposal of Hazardous  Material on or in the Installations  Premises during
the  Term  results  in the  release  or  threatened  release  of  Hazardous
Materials  at, on or under  the  Installations  Premises  in  violation  of
Applicable  Law,  or  otherwise  necessitates  investigation  or cleanup of
Hazardous  Material  as  required  under  Applicable  Law   ("Environmental
Conditions"),  Tenant shall  indemnify,  defend and hold Landlord  harmless
from any and all  claims,  judgments,  damages,  penalties,  fines,  costs,
liabilities  or  losses  (including,   without  limitation,  sums  paid  in
settlement of claims,  attorneys' fees, consultant fees and expert fees but
excluding  consequential  damages  and  any  injury  to  the  value  of the
Property,  provided that this clause shall not be construed as reducing the
Remediation  Obligation)  which arise  during or after the Term as a direct
result  thereof.  This  indemnification  of  Landlord  by Tenant  includes,
without  limitation,  reasonable  costs  incurred  in  connection  with any
investigation  of site  conditions  or any  cleanup,  remedial,  removal or
restoration  work  required  by any  federal,  state or local  governmental
agency because of Hazardous  Material present in the soil or groundwater on
or  under  the  Installations  Premises  or  in  any  improvements  on  the
Installations Premises.  This indemnification,  however, shall not apply to
any Environmental Conditions caused by the acts or omissions of
<PAGE>
 
Landlord or its Agents. The indemnification  and hold harmless  obligations
of Tenant under this Section 27 shall survive any termination of this Lease
for a period of twenty-four (24) months after the  termination.  At the end
of the  twenty-four  (24) month  period,  this  indemnification  and all of
Tenant's obligations under this Section shall expire,  except as to matters
specifically  made the subject of a lawsuit filed against Tenant before the
expiration  of the  twenty-four  (24) month  period.  Without  limiting the
foregoing,  if the use,  storage,  handling,  generation,  or  disposal  of
Hazardous  Material  on or in the  Installations  Premises  during the Term
results  in  any  Environmental   Conditions,   then,   provided  that  the
Environmental  Condition is not caused by the acts or omissions of Landlord
or its Agents, Tenant shall promptly take all actions, at its sole expense,
as are necessary to return the Installations  Premises to substantially the
condition  existing  prior to  thereto or to such  other  condition  as may
satisfy  the  applicable   governmental   authorities   (the   "Remediation
Obligation").  Landlord's approval of such actions shall first be obtained,
which approval shall not be unreasonably withheld.  Nothing in this Section
shall be deemed to prohibit or limit any action by Tenant against any party
or parties responsible for the contamination.

                  (b) Tenant  covenants  and agrees that during the Term it
shall  continue  implementation  of the  Operations  and  Maintenance  Plan
described in Exhibit J attached  hereto and made a part hereof at all times
in compliance with Applicable Laws in all material respects.

                  (c) Landlord shall have the right,  at any time, to cause
the groundwater,  soil,  improvements and air at the Installations Premises
to be investigated to detect the presence of Hazardous  Material during the
Term, including,  but not limited to, the installation of testing wells and
other  devices  in  locations  selected  by  Landlord  at  Landlord's  sole
discretion. Landlord shall supply Tenant with copies of final investigation
reports. The cost of such investigations and of the maintenance, repair and
replacement  of such  wells and other  devices  shall be fully  paid for by
Landlord,  unless Landlord's investigations reveal Environmental Conditions
which Tenant is obligated to remediate  under this Section.  In that event,
Tenant,   within   thirty  (30)  days  after   receiving  a  copy  of  such
investigation  report and a statement of charges from  Landlord,  shall pay
for the cost of the investigation.  Any dispute under this Subsection shall
be  resolved  under the  arbitration  proceedings  set forth in  Subsection
11(b).

                  (d) As used herein,  the term "Hazardous  Material" means
any  hazardous  or toxic  substance,  material or waste which is or becomes
regulated by any local governmental authority, the State of Maryland or the
United  States  Government  or other  Applicable  Law. The term  "Hazardous
Material" includes,  without limitation, any material or substance which is
(i)  designated as a "hazardous  substance"  pursuant to Section 307 of the
Federal  Water  Pollution  Control Act, 33 U.S.C.  Section 1251 et seq. (33
U.S.C.  Section  1317),  (ii)  defined as a "hazardous  waste"  pursuant to
Section 3001
<PAGE>
 
of the Federal Solid Waste Disposal Act, 42 U.S.C. Section 6901 et
seq. (42 U.S.C. Section 6921), or (iii) defined as a "hazardous
substance" pursuant to Section 101 of the Comprehensive
Environmental Response, Compensation and Liability Act, 42 U.S.C.
Section 9601 et seq. (42 U.S.C. Section 9601), or (iv) petroleum,
petroleum product, polychlorinated biphenyls or urea formaldehyde.

                  (e) Within 30 days of the  Expiration  Date, if requested
by Landlord, Tenant shall remove all fuel oil and other liquid contents and
Hazardous  Materials  (collectively,   the  "Contents")  from  all  now  or
hereafter  active  underground  storage tanks located on the  Installations
Premises and dispose of such contents off of the Installations Premises and
in  accordance  with  Applicable  Laws.  Tenant  shall not be  obligated to
remove, close or take any other action to de-commission the tanks during or
after the Term.  If,  however,  during  the Term  governmental  authorities
require such removal,  closure or other action and the  requirement  is not
prompted by Landlord's request to remove the Contents or any Development by
Landlord or its Agents,  Tenant shall  comply with such request  subject to
Subsection 5(b)(iv)(d).

                  (f)  Notwithstanding  anything  contained  in this Lease,
Tenant shall have no obligation or liability to Landlord with respect to:

                           (i)   Any Hazardous Material which may exist on the
Installations  Premises as of the Effective Date, except to the extent that
Tenant is  specifically  required to take action  regarding  such Hazardous
Material by the governmental authority having jurisdiction.

                           (ii)  Hazardous Material that migrates, flows,
percolates,  diffuses or in any way moves on to or under the  Installations
Premises from sources outside the Installations Premises.

                  SECTION 28.       RIGHT TO RENEW TERM

                  (a)  Provided  that this  Lease is then in full force and
effect,  and  provided  that no Event of Default  exists at the time of the
Renewal Notice or the date of  commencement  of the Renewal Term,  Landlord
hereby grants to the Tenant an option (the  "Renewal  Option") to renew the
Initial Term of this Lease on the same terms,  conditions and provisions as
contained in this Lease, except as noted herein, for a period of five years
after the Expiration Date of the Initial Term (the "Renewal  Term"),  which
Renewal Option period shall, except as provided below, commence immediately
following the expiration of the Initial Term and end at 11:59 p.m.
of the fifth anniversary of such date.

                  (b) The Renewal Option shall be exercised,  if at all, by
written  notice  (the  "Renewal  Notice")  from  Tenant to  Landlord of its
election,  said notice to be given no later than the  eighteen  (18) months
prior to the Expiration Date of the Initial Term. If


    
<PAGE>
 
the  Renewal  Notice  is not so given by Tenant to  Landlord,  the  Renewal
Option shall be deemed waived.

                  (c) The  Renewal  Term  shall  be upon  the  same  terms,
covenants and conditions as provided in this Lease except as follows:

                           (i)  Tenant shall not have any further right to
extend the Term or holdover after the end of the Term.

                           (ii) The annual Base Rent for the first year of the
Renewal  Term  shall be equal to  102.75%  of the Base  Rent rate in effect
under this Lease immediately prior to commencement of the Renewal Term.

                          (iii) The annual  Base Rent  shall be  increased
during the Renewal Term by 2.75% per year on a cumulative compounded
basis.

                  SECTION 29.       SECURITY DEPOSIT

                  If at any time  during  the Term,  the  credit  rating of
Tenant's  corporate  debt drops below BBB as rated by Standard & Poor's (or
the  equivalent  rating by any other national  rating agency  designated by
Landlord),  upon  notice from  Landlord,  Tenant  shall  deposit a sum with
Landlord  equal to one month's  installment of the then  applicable  annual
Base Rent  ("Security  Deposit"),  as  security  for the full and  faithful
performance by Tenant, of each and every term,  covenant,  and condition of
this Lease.  If there an event of Default,  Landlord  may use the  Security
Deposit as payment of any Rent or other payment due from Tenant to Landlord
or to otherwise  cure any default of Tenant  hereunder.  To the extent that
any of the Security Deposit is used for this purpose, Tenant shall pay such
amount to Landlord  along with the next  month's Rent in order to replenish
the Security  Deposit to the original  amount stated  herein.  The Security
Deposit shall be returned to Tenant within 30 days of the  Expiration  Date
or earlier  termination of this Lease to the extent that such amount is not
depleted in order to remedy any default by Tenant hereunder.

                  SECTION 30.       MISCELLANEOUS GENERAL PROVISIONS

                  (a) Payments Deemed Rent. Any amounts of money to be paid
by Tenant to Landlord pursuant to the provisions of this Lease,  whether or
not such payments are  denominated  Rent or Additional  Rent and whether or
not  they  are to be  periodic  or  recurring,  shall  be  deemed  Rent  or
Additional  Rent for purposes of this Lease;  and any failure to pay any of
same shall  entitle  Landlord  to exercise  all of the rights and  remedies
afforded  hereby or by law or in equity for the collection and  enforcement
of Tenant's  obligation  to pay Rent.  Tenant's  obligation to pay any such
Rent or  Additional  Rent  pursuant to the  provisions  of this Lease shall
survive the expiration or other termination of this Lease and the surrender
of possession of the Master Lease Property.
<PAGE>
 
                  (b)  Interest  on  Deposits.  Any amount  deposited  with
Landlord under this Lease shall be held by Landlord in a federally insured,
interest bearing account.  Any interest earned on such deposit shall accrue
to Tenant and shall be transferred to Tenant  promptly after the Expiration
Date or such earlier time as may be specified in this Lease unless Landlord
is required by  Applicable  Laws to return such  interest to Tenant  sooner
than stated  herein.  Any amount  deposited with Mortgagee by Tenant (or by
Landlord  on  Tenant's  behalf)  shall  be held by  Mortgagee  pursuant  to
Applicable Laws and pursuant to the security documents between Landlord and
Mortgagee.  No interest shall accrue unless  required by Applicable Laws or
such security documents.  Notwithstanding the foregoing, any amounts placed
in the Escrow  Account  shall be governed by the  provisions  of Subsection
21(b)(iii)(B).

                  (c)   Landlord Cross Default.  Any default by Landlord
under the Master Lease shall be a default by Landlord under this
Lease.

                  (d)  Estoppel  Letters.  Tenant  shall,  within  ten (10)
business days following written request from Landlord, execute, acknowledge
and  deliver to Landlord or to any then  existing  or  prospective  lender,
investor or purchaser,  with respect to the  Installations  Premises or any
part thereof,  designated by Landlord,  a written statement  certifying (i)
that  this  Lease is in full  force  and  effect  (if such is the case) and
unmodified (or, if modified, stating the nature of such modification), (ii)
the date to which Rent has been paid, (iii) that there are not, to Tenant's
actual knowledge, any uncured defaults by Landlord or Tenant (or specifying
such defaults if any are claimed),  and (iv) such other matters as Landlord
may reasonably  request.  Any such statement may be relied upon by any such
then existing or prospective lender, investor or purchaser. If Tenant fails
to  deliver  such  statement  within  the 10  business  day  period  and if
following the  expiration of that period  Landlord  gives a second  written
request for the statement and Tenant fails to deliver the statement  within
5 business  days after the second  request,  Tenant shall  conclusively  be
deemed to have  responded  that this  Lease is in full force and effect and
unmodified and that there are no uncured defaults in Landlord's performance
hereunder.

                  (e) Brokers.  Each party  represents  and warrants to the
other that no broker  procured this Lease on its behalf and that such party
had no conversations or negotiations with any broker concerning the leasing
of the  Installations  Premises,  other  than  Barnes,  Morris,  Pardoe and
Foster, which firm is receiving a commission from Tenant in connection with
the Purchase  and Sale  Agreement  and is not  entitled to a commission  in
connection  with this Lease.  Each party shall  indemnify the other against
liability in connection with a breach of its representation and warranty in
this  Subsection  and in  connection  with any  claim  for a  brokerage  or
finder's  commission or fee arising out of its acts.  This  indemnification
shall survive any termination of this Lease.
<PAGE>
 
                  (f) Applicable Law. This Lease and all matters  pertinent
thereto shall be construed and enforced in accordance  with the  Applicable
Laws of the State of Maryland, excluding choice of laws principles.

                  (g) Entire Agreement.  This Lease, including all exhibits
hereto,  constitutes the entire  agreement  between the parties hereto with
respect  to the  leasing  of the  Installations  Premises  and  may  not be
modified except by an instrument in writing executed by the parties hereto.

                  (h) Binding Effect.  This Lease and the respective rights
and  obligations of the parties hereto shall inure to the benefit of and be
binding upon the  successors  and assigns of the parties  hereto as well as
the parties themselves; subject, however, to Subsection (o) below.

                  (i) Survival. All provisions of this Lease which by their
express terms survive  termination  of this Lease or which by the operation
of their terms are  intended to be  performed,  in whole or in part,  after
termination of this Lease, shall survive any termination of this Lease.

                  (j) Severability. If any provision of this Lease shall be
held to be invalid,  void or unenforceable,  such provision shall be deemed
reformed  to be valid,  in effect  and  enforceable,  and to be as close in
meaning  and  intent as the  defective  provision  and still the  remaining
provisions  hereof  shall not be affected or impaired,  and such  remaining
provisions shall remain in full force and effect.

                  (k)  Headings,  Gender,  etc. As used in this Lease,  the
word "person" shall mean and include,  where  appropriate,  an indivi dual,
corporation,  partnership or other entity;  the plural shall be substituted
for the singular,  and the singular for the plural, where appropriate,  and
words of any gender shall include any other gender. The topical headings of
the  several  paragraphs  of this  Lease are  inserted  only as a matter of
convenience and reference, and do not affect, define, limit or describe the
scope or intent of this Lease.  References  in this Lease to  Sections  and
Subsections are references to Sections and Subsections of this Lease.

                  (l) Waiver of Jury. To the extent permitted by Applicable
Laws,  each of Landlord and Tenant hereby waives any right it may have to a
jury  trial  in  the  event  of  litigation  between  Landlord  and  Tenant
pertaining to this Lease.

                  (m) Landlord's Right to Cure. Landlord may, but shall not
be obligated to, cure any default by Tenant,  specifically  including,  but
not by way of  limitation,  Tenant's  failure  to pay  Impositions,  obtain
Insurance,  Care for the  Installations  Premises,  or satisfy lien claims,
after complying with any applicable notice and cure provisions  established
under this Lease;  and whenever  Landlord so elects,  all reasonable out of
pocket costs and expenses
<PAGE>
 
paid by Landlord in curing such  default,  including,  without  limitation,
reasonable  attorneys'  fees,  shall  be  Additional  Rent  due on the next
scheduled Rent payment date.

                  (n)  Relationship of Parties.  Nothing  contained  herein
shall be deemed or construed by the parties hereto, nor by any third party,
as creating the  relationship of principal and agent or of partnership,  or
of joint venture by the parties hereto, it being understood and agreed that
no  provision  contained  in this Lease nor any act of the  parties  hereto
shall be deemed to create any  relationship  other than the relationship of
Landlord and Tenant.

                  (o) Landlord Means Owner.  The term "Landlord" as used in
this lease,  so far as covenants or obligations on the part of Landlord are
concerned, shall be limited to mean and include only the owner or owners at
the time in question of the Installations Premises and, in the event of any
transfer or  transfers of the title to all of the  Installations  Premises,
Landlord  herein  named  (and  in  case  of  any  subsequent   transfer  or
conveyances,  the then grantor) shall be automatically  freed and relieved,
from and after the date of such transfer or conveyance, of all liability as
respects the  performance  of any covenants or  obligations  on the part of
Landlord  contained in this Lease  thereafter to be performed  (but not any
liabilities accrued prior to the date of transfer); provided that any funds
in the  hands  of such  Landlord  or the then  grantor  at the time of such
transfer,  in which  Tenant has an  interest,  shall be turned  over to the
grantee,  and any amount  then due and payable to Tenant by Landlord or the
then grantor under any  provisions  of this Lease,  shall be paid to Tenant
and  further  provided  that the  transferee  assumes in writing all of the
covenants  and  obligations  of Landlord to observed  and  performed on and
after the date of transfer.

                  (p) References to Size.  Landlord and Tenant  acknowledge
and agree that any  references  in this Lease to the size of the  Property,
the  Auxiliary  Building  Areas,  the  Auxiliary  Buildings  or any portion
thereof are for convenience  only and regardless of whether the actual size
of such areas is greater or less than the size  stated in this  Lease,  all
obligations of the parties  hereunder,  including without  limitation,  the
obligation  to pay Rent shall  remain the same and shall not be affected by
any errors in references to size.

                  (q) Unavoidable  Delays.  For purposes of this Lease, the
term  "Unavoidable  Delays" shall mean delays caused by strikes,  lockouts,
acts  of  God,  inability  to  obtain  labor  or  materials,   governmental
restrictions or inaction,  enemy action,  civil commotion,  fire, terrorist
action, epidemic, public utility failure, unavoidable casualty,  moratorium
or similar laws prohibiting  performance,  severe weather conditions or any
other  similar  matter  which  shall be beyond  the  reasonable  control of
Landlord or Tenant,  as the case way be; but the lack or  insufficiency  of
funds shall not constitute an Unavoidable Delay.
<PAGE>
 
                  (r) Landlord's Approvals.  Wherever Landlord's consent or
approval  are  required  under  this  Lease,   Landlord  shall  approve  or
disapprove  the matter within ten (10) business days after Tenant  requests
the  consent or approval  in  writing.  If Landlord  fails to do so and if,
following the expiration of the ten (10) business day period,  Tenant gives
a second written  request for the consent or approval and Landlord fails to
approve or  disapprove  the matter  within five (5) business days after the
second request,  Landlord shall conclusively be deemed to have consented to
or approved the matter, as the case may be.

                  (s) Rate of  Interest.  If any amount owed by Landlord to
Tenant under this Lease remains  unpaid after such amount is due and notice
thereof  has been given to  Landlord,  the  outstanding  amount  shall bear
interest at the  Stipulated  Rate from the date such amount is due and such
notice is given to the date such amount is paid.

                  (t)  Cooperation  of  Parties.  Whenever  the parties are
required to cooperate  with each other under this Lease but are entitled to
reimbursement  for their  out-of-pocket  costs to third parties,  the party
that is  obligated  to  cooperate  shall  provide an estimate of such third
party costs to the party  requesting such  cooperation and obtain the prior
written  approval from such other party not to be unreasonably  withheld or
delayed before such costs are incurred.

                  (u)  Reconciliation.  Nothing in this Lease shall prevent
or impair Tenant from performing its obligations or observing its covenants
under the  Existing  Space  Leases.  Further,  nothing in this Lease  shall
prevent or impair any of the Existing  Tenants from exercising their rights
and privileges under the Existing Space Leases. No such action by Tenant or
the Existing  Tenants in accordance with the Existing Space Leases shall be
deemed to be a breach or default by Tenant under this Lease.

                  (v) Memorandum of Lease. Upon request of either party the
other party shall promptly  execute and deliver a memorandum or other short
form  version of this  Lease  setting  forth the basic  terms of this Lease
excluding Rent. The party recording such memorandum, short form version, or
other  document  giving  notice  of  this  Lease  shall  pay  any  and  all
recordation and transfer taxes due in connection with such recordation.

                  (w)  Financial  Reports.  In the event that  Tenant is no
longer a publicly  traded  company with common stock  trading on either the
New York Stock  Exchange,  the American  Stock Exchange or the NASDAQ Stock
Exchange  (or, in the event the Tenant's  stock is no longer traded on such
exchanges, or a successor or reasonably equivalent exchange),  Tenant shall
supply  Landlord,  within  ten (10)  business  days of  Landlord's  request
therefor (to be no more  frequent  than once per year),  copies of Tenant's
most recent  financial  reports.  Such reports  shall (i) include an income
statement, balance sheet, statement owner's equity and statement of


 
<PAGE>
 
cash flows,  (ii) shall be audited by a certified public  accountant or, to
the extent audited Financial  Reports are not otherwise  obtained by Tenant
for other purposes,  certified by the chief financial officer of Tenant, to
his/her  knowledge,  as being true, correct and complete financial reports,
and (iii) shall be dated no later than twelve  months  prior to  Landlord's
request. Notwithstanding the foregoing, in the event that Landlord requests
such financial  reports within one hundred twenty (120) days after Tenant's
fiscal year end, Tenant shall have up to one hundred twenty (120) days from
such fiscal year end to supply such  reports to Landlord;  provided  Tenant
has given Landlord its most recent  financial  reports and such reports are
not dated  earlier  than twelve (12) months  prior to Tenant's  fiscal year
end.

                  (x)  Landlord's  Affiliates.   Notwithstanding   anything
contained  in  this  Lease  to the  contrary,  any  act or  omission  of an
affiliate  of  Landlord  on any  Excluded  Areas  shall not be a default of
Landlord under this Lease. In no event shall this provision be construed in
any  manner as a waiver of any right  that  Tenant  has at law or in equity
against such affiliate as a result of such act or occurrence.



                          [SIGNATURE PAGE FOLLOWS]
<PAGE>
 
                  IN WITNESS  WHEREOF,  the parties hereto have caused this
Lease to be executed as of the day and year stated herein.

                                 LANDLORD:

                                 LCOR CLARKSBURG L.L.C.
                                 By: Clarksburg Management, Inc.


                                 By: /s/ Michael T. Goulder

                                 Name: Michael T. Goulder

                                 Title: Vice President



                                 TENANT:

                                 COMSAT CORPORATION


                                 By: /s/ Allen E. Flower

                                 Name: Allen E. Flower

                                 Title: Vice President and
                                        Chief Financial Officer
<PAGE>
 
                                 SCHEDULE I



         AAA:                       Section 11(b)
         Accelerated Payment:       Section 21(b)(iii)(A)
         Additional Easements:      Section 6(f)
         Additional Rent:           Section 4(b)
         Adequate Assurance of
           Future Performance:      Section 22(d)
         All Risks:                 Section 14(a)(i)
         Alterations:               Section 13(a)
         Applicable Laws:           Section 5(b)(iv)
         Appurtenances:             Section 6(d)
         Assignment:                Section 19(b)
         Base Rent:                 Section 4(a)
         Care, Care for,
           Care of:                 Section 11(a)
         Comparable Properties:     Section 14(a)(ii)
         Contents:                  Section 27(e)
         Damage:                    Section 15(a)
         Declaration:               Section 11(a)
         Default Measuring
           Period:                  Section 21(b)(iii)(B)
         Development:               Section 6(a)
         Effective Date:            Preamble
         Environmental
           Conditions:              Section 27(a)
         Escrow Account:            Section 21(b)(iii)(B)
         Escrowee:                  Section 21(b)(iii)(B)
         Events of Default:         Section 21(a)
         Excluded Area:             Section 6(d)
         Existing Space Leases:     Section 19(a)
         Existing Tenants:          Section 19(a)
         Hazardous Material:        Section 27 and 27(d)
         Holdover Period:           Section 24
         Impositions:               Section 7(a) and 7(d)
         Indemnifying Party:        Section 16(c)
         Indemnitee:                Section 16(c)
         Indemnity Payment:         Section 16(c)
         Initial Term:              Section 3(a)
         Insurance:                 Section 14(a)
         Insurance Notice:          Section 14(g)
         Intelsat:                  Section 13(c)
         Landlord:                  Preamble, Section 30(o)
         Landlord Approved SNDA:    Section 19(b)
         Lease:                     Preamble
         Lease Year:                Section 3(b)
         Liability Policy:          Section 14(a)(ii)
         Master Lease:              Section 21(a)(iii)
         Mitigation Leases:         Section 21(b)(iii)(B)
         Mortgagee:                 Section 7(e)(i)
         Net Award:                 Section 17(b)
         Operating Expenses:        Section 21(b)(iii)(B)
<PAGE>
 
         Post Default Leases:       Section 21(b)(iii)(B)
         Prime Rate:                Section 4(h)
         Purchase and Sale
           Agreement:               Recital (a)
         Remediation Obligation:    Section 27(a)
         Renewal Notice:            Section 28(b)
         Renewal Option,
           Renewal Term:            Section 28(a)
         Rent:                      Section 4(b)
         Restricted Areas:          Section 12
         Restricted Property:       Section 13(c)
         Security Deposit:          Section 29
         Signs:                     Section 9(a)
         Stipulated Rate:           Section 4(h)
         Sublease:                  Section 19(b)
         Subtenant:                 Section 19(c)
         Taken, Taking:             Section 17(a)
         Tax and Insurance
           Holdbacks:               Section 21(b)(iii)(B)
         Tenant:                    Preamble
         Tenant Approved SNDA:      Section 20(a)
         Tenant's Property:         Section 13(c)
         Term:                      Section 3(a)
         Third Party Claim:         Section 16(d)(i)
         Unavoidable Delays:        Section 30(q)
         Utilities:                 Section 8(a)
<PAGE>
 
                            SCHEDULE OF EXHIBITS



                  EXHIBIT                                         SECTION

                  Exhibit A    (Description of Land)              1(a)

                  Exhibit B    (Description of                    1(a)
                                     Main Building Area)

                  Exhibit C    (Description of                    1(a)
                                Installations Premises)

                  Exhibit D    Intentionally Omitted

                  Exhibit E    (Wire Instructions)                4(g)

                  Exhibit F    (Portion of Installations
                                Premises)                        17(c)

                  Exhibit G    (Existing Space Leases)           19(a)

                  Exhibit H    (Landlord Approved SNDA)          19(b)

                  Exhibit I    (Tenant Approved SNDA)            20(a)

                  Exhibit J    (Operations and Maintenance       27(b)
                                Plan)